R1:adb1d3a5be68

Fix autocomplete/send-on-enter interactions

Summary:
Send-on-enter and autocomplete both listen for "return" keypresses, and could race. Have autocomplete let other handlers take a shot at the action before it does.

Also, fix a case where ":)" and the suffix list (which lets you type `someone is 100% to blame here (@epriestley)` and get the results you want) interacted badly, so ":)" cancels the autocompleter like ":3" does.

Test Plan:
- Typed "@xxx" and mashed return real fast over and over again…