Modularize mail tags
Summary:
Ref T5861. Currently, mail tags are hard-coded; move them into applications. Each Editor defines its own tags.
This has zero impact on the UI or behavior.
Test Plan:
- Checked/unchecked some options, saved form.
- Swapped back to master and saw exactly the same values.
Reviewers: chad, btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T5861
Differential Revision: https://secure.phabricator.com/D10238