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