Make MP4s Autoplay and Loop in the Timeline

MP4 videos in the timeline should be set to autoplay and loop, like how they are set in the post composer preview compose-form__upload-preview class

MP4s can be short clips like GIFs or they can be longer. I think that most of the MP4 attachments are short clips.

The autoplay and loop defaults should not affect users viewing long MP4 videos, as they will have moved to a different part of the timeline before autoplay/looping becomes an issue.

Some systems, like Telegram, allow downloading of GIFs, but they are downloaded as MP4s, then must be externally converted to GIFs currently, in order to autoplay and loop in the timeline.