Summary:
- Detecting emoji is surprisingly tricky; we use a fancy regex as a decent heuristic.
- Icons are a little larger than before.
- Styling tweaked for light and dark modes
- In case the OS doesn't render the emoji as one character, truncate what's
shown in the icon box.
Test Plan: Added a test case.
Reviewers: jarek
Reviewed By: jarek
Differential Revision: https://phab.getgrist.com/D3904