Use vanniktech emoji library

I know I'm not supposed to be working on this, but it's world emoji day.

As far as I can tell, the implementation does everything the old implementation did, with less code for us to maintain. But it makes us dependent on vanniktech for updates. WIP so we can discuss whether this is the right approach.

Fixes #1343 (closed), #1314 (closed), #940 (closed), #930 (closed), #749 (closed), and #684 (closed), but not #751 or #750.