Create video files instead of GIFs

GIFs are very large. [Imgur stopped using actual GIFs in 2014 due to the large file size](https://blog.imgur.com/2014/10/09/introducing-gifv/). This is part of what is causing performance issues for Thicket (see #14 and #15).

It would be nice to use a video format instead, which gives higher quality at smaller file sizes.

### Possible libraries

* https://github.com/muaz-khan/RecordRTC

See other options on [this StackOverflow post](https://stackoverflow.com/questions/18509385/html-5-video-recording-and-storing-a-stream)