FEATURE: Initial implementation of direct S3 uploads with uppy and stubs (#13787)

This adds a few different things to allow for direct S3 uploads using uppy. **These changes are still not the default.** There are hidden `enable_experimental_image_uploader` and `enable_direct_s3_uploads`  settings that must be turned on for any of this code to be used, and even if they are turned on only the User Card Background for the user profile actually uses uppy-image-uploader.

A new `ExternalUploadStub` model and database table is introduced in this pull request. This is used to keep track of uploads that are uploaded to a temporary location in S3 with the direct to S3 code, and they are eventually deleted a) when the direct upload is completed and b) after a certain time period of not being used. 

### Starting a direct S3 upload

When an S3 direct upload is initiated with uppy, we first request a presigned PUT URL from the new `generate-presigned-put` endpoint in `UploadsController`. This generates an S3 key in the `temp` folder inside the correct bucket path, along with any metadata from the clientside (e.g. the SHA1 checksum described below). This will also create an `ExternalUploadStub` and store the details of the temp object key and the file being uploaded.

Once the clientside has this URL, uppy will upload the file direct to S3 using the presigned URL. Once the upload is complete we go to the next stage.

### Completing a direct S3 upload

Once the upload to S3 is done we call the new `complete-external-upload` route with the unique identifier of the `ExternalUploadStub` created earlier. Only the user who made the stub can complete the external upload. One of two paths is followed via the `ExternalUploadManager`.

1. If the object in S3 is too large (currently 100mb defined by `ExternalUploadManager::DOWNLOAD_LIMIT`) we do not download and generate the SHA1 for that file. Instead we create the `Upload` record via `UploadCreator` and simply copy it to its final destination on S3 then delete the initial temp file. Several modifications to `UploadCreator` have been made to accommodate this.

2. If the object in S3 is small enough, we download it. When the temporary S3 file is downloaded, we compare the SHA1 checksum generated by the browser with the actual SHA1 checksum of the file generated by ruby. The browser SHA1 checksum is stored on the object in S3 with metadata, and is generated via the `UppyChecksum` plugin. Keep in mind that some browsers will not generate this due to compatibility or other issues.

    We then follow the normal `UploadCreator` path with one exception. To cut down on having to re-upload the file again, if there are no changes (such as resizing etc) to the file in `UploadCreator` we follow the same copy + delete temp path that we do for files that are too large.

3. Finally we return the serialized upload record back to the client

There are several errors that could happen that are handled by `UploadsController` as well.

Also in this PR is some refactoring of `displayErrorForUpload` to handle both uppy and jquery file uploader errors.

lib/file_store/s3_store.rb

@@ -35,6 +35,22 @@ module FileStore
       url
     end
 
+    def move_existing_stored_upload(existing_external_upload_key, upload, content_type = nil)
+      upload.url = nil
+      path = get_path_for_upload(upload)
+      url, upload.etag = store_file(
+        nil,
+        path,
+        filename: upload.original_filename,
+        content_type: content_type,
+        cache_locally: false,
+        private_acl: upload.secure?,
+        move_existing: true,
+        existing_external_upload_key: existing_external_upload_key
+      )
+      url
+    end
+
     def store_optimized_image(file, optimized_image, content_type = nil, secure: false)
       optimized_image.url = nil
       path = get_path_for_optimized_image(optimized_image)
@@ -42,10 +58,18 @@ module FileStore
       url
     end
 
+    # File is an actual Tempfile on disk
+    #
+    # An existing_external_upload_key is given for cases where move_existing is specified.
+    # This is an object already uploaded directly to S3 that we are now moving
+    # to its final resting place with the correct sha and key.
+    #
     # options
     #   - filename
     #   - content_type
     #   - cache_locally
+    #   - move_existing
+    #   - existing_external_upload_key
     def store_file(file, path, opts = {})
       path = path.dup
 
@@ -72,7 +96,16 @@ module FileStore
       path.prepend(File.join(upload_path, "/")) if Rails.configuration.multisite
 
       # if this fails, it will throw an exception
-      path, etag = s3_helper.upload(file, path, options)
+      if opts[:move_existing] && opts[:existing_external_upload_key]
+        path, etag = s3_helper.copy(
+          opts[:existing_external_upload_key],
+          path,
+          options: options
+        )
+        s3_helper.delete_object(opts[:existing_external_upload_key])
+      else
+        path, etag = s3_helper.upload(file, path, options)
+      end
 
       # return the upload url and etag
       [File.join(absolute_base_url, path), etag]
@@ -162,10 +195,14 @@ module FileStore
 
     def url_for(upload, force_download: false)
       upload.secure? || force_download ?
-        presigned_url(get_upload_key(upload), force_download: force_download, filename: upload.original_filename) :
+        presigned_get_url(get_upload_key(upload), force_download: force_download, filename: upload.original_filename) :
         upload.url
     end
 
+    def path_from_url(url)
+      URI.parse(url).path.delete_prefix("/")
+    end
+
     def cdn_url(url)
       return url if SiteSetting.Upload.s3_cdn_url.blank?
       schema = url[/^(https?:)?\/\//, 1]
@@ -175,7 +212,21 @@ module FileStore
 
     def signed_url_for_path(path, expires_in: S3Helper::DOWNLOAD_URL_EXPIRES_AFTER_SECONDS, force_download: false)
       key = path.sub(absolute_base_url + "/", "")
-      presigned_url(key, expires_in: expires_in, force_download: force_download)
+      presigned_get_url(key, expires_in: expires_in, force_download: force_download)
+    end
+
+    def signed_url_for_temporary_upload(file_name, expires_in: S3Helper::UPLOAD_URL_EXPIRES_AFTER_SECONDS, metadata: {})
+      key = temporary_upload_path(file_name)
+      presigned_put_url(key, expires_in: expires_in, metadata: metadata)
+    end
+
+    def temporary_upload_path(file_name)
+      path = super(file_name)
+      s3_bucket_folder_path.nil? ? path : File.join(s3_bucket_folder_path, path)
+    end
+
+    def object_from_path(path)
+      s3_helper.object(path)
     end
 
     def cache_avatar(avatar, user_id)
@@ -248,7 +299,19 @@ module FileStore
 
     private
 
-    def presigned_url(
+    def presigned_put_url(key, expires_in: S3Helper::UPLOAD_URL_EXPIRES_AFTER_SECONDS, metadata: {})
+      signer = Aws::S3::Presigner.new(client: s3_helper.s3_client)
+      signer.presigned_url(
+        :put_object,
+        bucket: s3_bucket_name,
+        key: key,
+        acl: "private",
+        expires_in: expires_in,
+        metadata: metadata
+      )
+    end
+
+    def presigned_get_url(
       url,
       force_download: false,
       filename: false,
@@ -262,7 +325,7 @@ module FileStore
         )
       end
 
-      obj = s3_helper.object(url)
+      obj = object_from_path(url)
       obj.presigned_url(:get, opts)
     end
 
@@ -276,7 +339,7 @@ module FileStore
 
     def update_ACL(key, secure)
       begin
-        s3_helper.object(key).acl.put(acl: secure ? "private" : "public-read")
+        object_from_path(key).acl.put(acl: secure ? "private" : "public-read")
       rescue Aws::S3::Errors::NoSuchKey
         Rails.logger.warn("Could not update ACL on upload with key: '#{key}'. Upload is missing.")
       end