R1:5e6716399cd6

Make Settings modular and allow them to be EditEngine'd

Summary: Ref T4103. This starts breaking out settings in a modern way to prepare for global defaults.

Test Plan:
- Edited diff settings.
- Saw them take effect in primary settings pane.
- Set stuff to new automatic defaults.
- Tried to edit another user's settings.
- Edited a bot's settings as an administrator.

{F1669077}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.