This doesn't work on two grounds:
* There is significant pixelation
* The resulting image size is wrong (but the frames are correct)

This is part of #28 and needs some cleaning up before it's an active code path. Pushing now because this code path won't be activated under normal circumstances.

This is the first part of #28. It defaults to false to maintain synapse parity and normal expected behaviour. This commit does the basic gif->png thumbnailing (forced when required), but does not handle resizing gifs.

Adds #27

Crop causes the thumbnail to become insanely large in Riot. For synapse parity, we'll use scale by default.

This will make more sense when the resource locks come into play - they'll need a place to live.

Currently not used though

Fixes #25 and https://github.com/vector-im/riot-web/issues/2585 (when using matrix-media-repo)

Fixes #26

Adds #6

Adds #16

This is done over http to ensure that synapse is left with a backup of the media repository. 

Adds #4

Adds #21

Otherwise clients would end up hitting us with more requests - we'll just reuse the existing URI.

Adds #17

Adds #8

500 errors are automatically handled by the handlers - if something serious goes wrong outside of the handler, we're screwed anyways.