Link external bug trackers

Summary: Fixes T2971.

Test Plan:
/rG1.
Set regexp to one line and URL, then /rG1.
Set regexp to two lines, then /rG1.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2971

Differential Revision: https://secure.phabricator.com/D5676