Gut ManiphestTransactionEditor

Summary:
Ref T2217. Removes most of the code from ManiphestTransactionEditor.

• Provides mail tag support in ManiphestTransactionEditorPro.
• There was one more write (subscribe/unsubscribe button) that I'd missed; modernize that.

Test Plan:

• Clicked subscribe/unsubscribe.
• Made some edits, verified mail had appropriate mail tags.

Reviewers: btrahan, garoevans

Reviewed By: garoevans

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7091