ugoira: store frame delays in MediaMetadata model.

Store Ugoira frame delays in the MediaMetadata model as a fake EXIF
field instead of in the PixivUgoiraFrameData model. This way we can get
rid of the PixivUgoiraFrameData model completely. This is a step towards
fixing #5264.

app/logical/media_file/ugoira.rb

@@ -22,6 +22,10 @@ class MediaFile::Ugoira < MediaFile
     preview_frame.close
   end
 
+  def metadata
+    super.merge("Ugoira:FrameDelays" => frame_delays)
+  end
+
   def dimensions
     preview_frame.dimensions
   end