Add video support from Twitter embeds + Remove `u-photo` from Twitter photos

As found with #883 (closed), a Tweet from Tech Nottingham included a GIF, which under the video portion of the MF2 JSON, not the

An assumption we can make here is that the MIME type is going to be an MP4 for everything, but that isn't ideal.

GIFs make sense to loop, and we want to limit the size of the video element on the page, especially as they're not all built for size.

Closes #883 (closed).