A couple of bugs have cropped up with viewing of mp4 gifs. This hasn't
been enabled internally, so I'm guessing some bugs have snuck through as
other changes have been made in the vicinity. This addresses most of
them:
- Better layout in the media detail viewer
- More accurate descriptions
- Fix for failed thumbnailing
- Fix for keyboard tracking bug with reduce motion enabled
- Fetch mp4s from Giphy for showing previews
- Fetch mp4s from Giphy for sending (behind featuree flag)
- A new flag on attachments to tag a video as "looping media"
- Videos tagged with this flag will be shown on repeat
