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.com/D15995