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