Summary:
Emojis can now be added in the composer window with colons and the emoji names (referenced by the same names used on Slack/GitHub/etc.).
When using the correct syntax, if there are emojis that match the text typed, they appear in a dropdown floating toolbar.
Selection works with either mouse clicks or arrow keys (plus `Enter`).
Currently, the toolbar won't trigger if the colon is adjacent to a non-whitespace character.
Test Plan: TODO: Will write tests soon!
Reviewers: evan, bengotow
Reviewed By: bengotow
Differential Revision: https://phab.nylas.com/D2505
