R1:7ef6c0a523fc

Modularize all the mail preferences

Summary:
Ref T4103. This isn't completely perfect but should let us move forward without also expanding scope into "too much mail".

I split the existing "Mail Preferences" into two panels: a "Mail Delivery" panel for the EditEngine settings, and a "2000000 dropdowns" panel for the two million dropdowns. This one retains the old code more or less unmodified.

Test Plan:
- Ran unit tests, which cover most of this stuff.
- Grepped for all removed constants.
- Ran…