Animated GIF and Video support

BUG: 463903

BUG: 463369

There are two separate bugs requesting video and GIF support. I'm pretty sure that GIFs work, but don't auto-play on the timeline.

• Check whether or not GIFs actually play at all
• Add option to enable/disable auto-playing GIFs on the timeline (this is a local option, not stored as a Mastodon preference)
• Add video support (embed and attachment?)