Automatically link PHIxxx on "secure" to Support on Admin

Summary: Ref T12904. This just makes PHI123 autolink on secure. No hovercard / mention / fancy cross-install behavior, but should improve quality of life a little going forward.

Test Plan:

Reviewers: chad, amckinley

Reviewed By: chad

Maniphest Tasks: T12904

Differential Revision: https://secure.phabricator.com/D18222