FEATURE: allow specifying s3 config via globals

This refactors handling of s3 so it can be specified via GlobalSetting

This means that in a multisite environment you can configure s3 uploads
without actual sites knowing credentials in s3

It is a critical setting for situations where assets are mirrored to s3.

spec/components/file_store/s3_store_spec.rb

@@ -16,6 +16,7 @@ describe FileStore::S3Store do
     SiteSetting.s3_upload_bucket = "s3-upload-bucket"
     SiteSetting.s3_access_key_id = "s3-access-key-id"
     SiteSetting.s3_secret_access_key = "s3-secret-access-key"
+    SiteSetting.enable_s3_uploads = true
   end
 
   shared_context 's3 helpers' do