posts: mark media asset as expunged when post is expunged.

Fix it so that when a post is expunged, the media asset is also marked
as expunged. This way the files will be deleted, but the media asset
will still remain as a record of what was expunged. The media asset will
have the md5, width, height, file ext, and file size of the deleted file.

app/models/media_asset.rb

@@ -179,6 +179,14 @@ class MediaAsset < ApplicationRecord
       self.pixiv_ugoira_frame_data = PixivUgoiraFrameData.new(data: media_file.frame_data, content_type: "image/jpeg") if is_ugoira?
     end
 
+    def expunge!
+      delete_files!
+      update!(status: :expunged)
+    rescue
+      update!(status: :failed)
+      raise
+    end
+
     def delete_files!
       variants.each(&:delete_file!)
     end