StorageManager: remove Post-specific code.

Refactor StorageManager to remove all image URL generation code. Instead
the image URL generation code lives in MediaAsset.

Now StorageManager is only concerned with how to read and write files to
remote storage backends like S3 or SFTP, not with how image URLs should
be generated. This way the file storage code isn't tightly coupled to
posts, so it can be used to store any kind of file, not just images
belonging to posts.

app/models/media_asset.rb

@@ -38,7 +38,9 @@ class MediaAsset < ApplicationRecord
     end
 
     def open_file
-      storage_service.open(file_path)
+      file = storage_service.open(file_path)
+      frame_data = media_asset.pixiv_ugoira_frame_data&.data if media_asset.is_ugoira?
+      MediaFile.open(file, frame_data: frame_data)
     end
 
     def convert_file(media_file)