Don't show the autocompleter until the user types at least one character

Summary: Ref T10163. Activate on @d, not just @. Note that if you type @d and then press delete once so you're left with @, we stay active (and show the "type a username" hint).

Test Plan:

• Typed @, no completer.
• Typed d, got completer.
• Typed delete, still had completer, now showing hint prompt.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10163

Differential Revision: https://secure.phabricator.com/D15037