WebM support

Allow uploading webm files. Lately webm has been becoming popular as an animation format on various sites, since it has better quality than gif and better file size than gif or apng.

One thing it would be immediately useful for is as a way of uploading [pixiv ugoira format](https://github.com/r888888888/danbooru/issues/2212). Though webm is useful besides that as well.

Note: this implementation includes thumbnail generation for webms as well.