Immediately cancel autocomplete if the user types certain characters anywhere

Summary:
Ref T10163. Some characters are almost certainly punctuation or markup rather than autocomplete requests. Immediately cancel any active autocomplete when the user types one of these.

Note that some of these are also suffix characters. If you type @dog,, you have until the next character to decide you actually mean to autocomplete. Once you type something else we deactivate.

If you type #dog# or ##, we deactivate immediately.

Test Plan: Typed @dog#, etc.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10163

Differential Revision: https://secure.phabricator.com/D15036