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.

test/factories/post.rb

@@ -25,8 +25,7 @@ FactoryBot.define do
       image_height { media_file.height }
       file_ext { media_file.file_ext }
       file_size { media_file.file_size }
-
-      before(:create) { |post, evaluator| MediaAsset.upload!(evaluator.media_file) }
+      media_asset { MediaAsset.upload!(media_file) }
     end
   end
 end