R1:5227a06e3c0e

Make Differential's username regex match People's username regex. Refs T3264.

Test Plan: Tested on our local phabricator install.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T3264

Differential Revision: https://secure.phabricator.com/D6066