R1:2f694f5e3f58

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.