Added willEditTask and didEditTask for Differential.

Summary: Added constants to PhabricatorEventType. Modified DifferentialRevisionEditor and DifferentialCommentEditor.

Test Plan:
Created a revision. Edited and made a comment on that revision. It's updating as usual. I think nothing broke may be it's working.
Let me know if I have done it correclty.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan

Maniphest Tasks: T2899

Differential Revision: https://secure.phabricator.com/D5869