Support animated GIF thumbnails

[matrixbot]

This issue was originally created by @0x1a8510f2 at matrix-org/dendrite#3282.

Description:

I recently added some animation to my avatar, and was incredibly sad to find out I couldn't enjoy it in all its glory on Matrix because I run Dendrite.

matrix-media-repo supports GIF thumbnails but is overkill for my small homeserver.

It would be super cool if this could be natively supported by Dendrite.

Some inspiration could probably be taken from the aforementioned: https://github.com/turt2live/matrix-media-repo/blob/master/thumbnailing%2Fi%2Fgif.go