FEATURE: Add support for secure media (#7888)

This PR introduces a new secure media setting. When enabled, it prevent unathorized access to media uploads (files of type image, video and audio). When the `login_required` setting is enabled, then all media uploads will be protected from unauthorized (anonymous) access. When `login_required`is disabled, only media in private messages will be protected from unauthorized access. 

A few notes: 

- the `prevent_anons_from_downloading_files` setting no longer applies to audio and video uploads
- the `secure_media` setting can only be enabled if S3 uploads are already enabled and configured
- upload records have a new column, `secure`, which is a boolean `true/false` of the upload's secure status
- when creating a public post with an upload that has already been uploaded and is marked as secure, the post creator will raise an error
- when enabling or disabling the setting on a site with existing uploads, the rake task `uploads:ensure_correct_acl` should be used to update all uploads' secure status and their ACL on S3

spec/mailers/user_notifications_spec.rb

@@ -674,6 +674,50 @@ describe UserNotifications do
     expect(mail.body.to_s).to match(I18n.t("user_notifications.reached_limit", count: 2))
   end
 
+  describe "secure media" do
+    let(:video_upload) { Fabricate(:upload, extension: "mov") }
+    let(:user) { Fabricate(:user) }
+    let(:post) { Fabricate(:post) }
+
+    before do
+      SiteSetting.s3_upload_bucket = "some-bucket-on-s3"
+      SiteSetting.s3_access_key_id = "s3-access-key-id"
+      SiteSetting.s3_secret_access_key = "s3-secret-access-key"
+      SiteSetting.s3_cdn_url = "https://s3.cdn.com"
+      SiteSetting.enable_s3_uploads = true
+      SiteSetting.secure_media = true
+      SiteSetting.login_required = true
+
+      video_upload.update!(url: "#{SiteSetting.s3_cdn_url}/#{Discourse.store.get_path_for_upload(video_upload)}")
+      user.email_logs.create!(
+        email_type: 'blah',
+        to_address: user.email,
+        user_id: user.id
+      )
+    end
+
+    it "replaces secure audio/video with placeholder" do
+      reply = Fabricate(:post, topic_id: post.topic_id, raw: "Video: #{video_upload.url}")
+
+      notification = Fabricate(
+        :notification,
+        topic_id: post.topic_id,
+        post_number: reply.post_number,
+        user: post.user,
+        data: { original_username: 'bob' }.to_json
+      )
+
+      mail = UserNotifications.user_replied(
+        user,
+        post: reply,
+        notification_type: notification.notification_type,
+        notification_data_hash: notification.data_hash
+      )
+
+      expect(mail.body.to_s).to match(I18n.t("emails.secure_media_placeholder"))
+    end
+  end
+
   def expects_build_with(condition)
     UserNotifications.any_instance.expects(:build_email).with(user.email, condition)
     mailer = UserNotifications.public_send(