Hide the autocompleter intelligently when you ignore it and keep typing

Summary:
Ref T10163. When we think the user has finished typing a word (because they typed a space, period, or other similar characters) and nothing else they might type could possibly change the outcome (usually because the words they have typed already match nothing), just deactivate the autocomplete.

As a special case, if the word they have typed already select exactly one result, and they have already typed exactly that result, assume they just typed it from memory and deactivate.

Test Plan:

• Typed @dog qwer zxcv and saw autocomplete deactivate on the space before z (on my local install, @dog is ambiguous but @dog qwer matches nothing).
• Typed @epriestley and saw autocomplete deactivate on space.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10163

Differential Revision: https://secure.phabricator.com/D15039