When autocompleting, add a space if a user didn't already add something

Summary: Ref T10163. When a user autocompletes, add a space for them, unless they already added a comma, colon, or space themsevles.

Test Plan: Autocompleted @dog, got @dog . Autocompleted @epriestley, got @epriestley,.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10163

Differential Revision: https://secure.phabricator.com/D15041