Attach mentioned Maniphest task also to revision

Summary: Also disable this feature without 'maniphest.enabled'.

Test Plan:
Wrote "fixes T..." in arc diff, verified that the task is attached.
Add another "fixes T..." in edit revision on web, verified that it was added.
Deleted "fixes T..." in edit revision, verified that the attached task wasn't deleted.

Reviewers: 20after4, epriestley

Reviewed By: 20after4

CC: aran, Korvin

Maniphest Tasks: T945

Differential Revision: https://secure.phabricator.com/D3986