R1:2b9d4f70ba3a

Remarkup rule for rendering PHIDs as handles

Summary:
adds the `{{PHID....}}` rule. Should mostly be useful in UI code that refers to Objects.

It doesn't add any mention links/transactions.

Test Plan: Comment with this, see email (plain + html) and comment box.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15488