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.

• Ran unit tests, which cover most of this stuff.
• Grepped for all removed constants.
• Ran migrations, inspected database results.
• Changed settings in both modified panels.
• This covers a lot of ground, but anything I missed will hopefully be fairly obvious.