R1:fa2467e8fe2e

Extract out regex from PhabricatorRemarkupRuleMention

Summary:
I'd like to use this regex elsewhere and copying and pasting is
bad.

Test Plan: none

Reviewers: casey, epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2038