Change PhabricatorPhurlURLViewController to use EditEngine for commenting
Test Plan: Created a phurl, added some comments, confirmed that "Change Subscribers" and "Change Project Tags" are now available in the comment form.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: chad, Korvin
Maniphest Tasks: T11661
Differential Revision: https://secure.phabricator.com/D17686