media assets: move more file-handling logic into MediaAsset.

Move more of the file-handling logic from UploadService and
StorageManager into MediaAsset. This is part of refactoring posts and
uploads to allow multiple images per post.

app/models/post.rb

@@ -1194,7 +1194,7 @@ class Post < ApplicationRecord
         ModAction.log("<@#{user.name}> regenerated IQDB for post ##{id}", :post_regenerate_iqdb, user)
       else
         media_file = MediaFile.open(file, frame_data: pixiv_ugoira_frame_data&.data.to_a)
-        UploadService::Utils.process_resizes(self, nil, id, media_file: media_file)
+        media_asset.distribute_files!(media_file)
 
         update!(
           image_width: media_file.width,