Modularize all straightforward settings

Summary:
Ref T4103. This tackles all the easy stuff. Not yet handled:

• Translation, pronoun, timezone: these are weird and stored on the User object instead of in settings.
• Conpherence default: actually just missed this one, it's normal.
• 1000 dropdowns for email notification preferences (messy, technically).

Test Plan:
wow look at all these settings

{F1670442}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D15999