Make a PhabricatorRemarkupControl to de-duplicate code usage around adding a Remarkup reference to a TextAreaControl.

Summary: ...also makes Maniphest Task Edit Controller use this when its not appropriate to upsell email.

Test Plan: played around with each tool and verified the Remarkup reference was present

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1756

Differential Revision: https://secure.phabricator.com/D3468