Make Pholio description behave as a remarkup field (e.g., subscribe mentioned users)

Summary: Ref T12732. This is pre-existing but fix it since I caught it while banging around.

Test Plan:

Reviewers: chad, amckinley

Reviewed By: chad

Maniphest Tasks: T12732

Differential Revision: https://secure.phabricator.com/D17970