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 while reloading the page. Before: sometimes handlers raced and text submitted. After: always handled by autocomplete behavior.
• Typed ":", ")", "<return>", sent an emoticon (previously: no).

Reviewers: chad, amckinley

Reviewed By: chad

Subscribers: xxx

Differential Revision: https://secure.phabricator.com/D17794