Support GIF effects in the web client

[floe]

Options (in decreasing order of feasibility):

• https://github.com/blindman67/GIFGroover
• https://github.com/themadcreator/gifler
• https://github.com/benwiley4000/gif-frames
• https://github.com/matt-way/gifuct-js
• https://github.com/buzzfeed/libgif-js

[floe]

Note: GIFs work in the web frontend itself, but the outgoing stream will always only show the very first frame of the GIF.