R1:67482fd19d2c

Continue modernizing application access to user preferences

Summary:
Ref T4103. This is just incremental cleanup:

- Add "internal" settings, which aren't editable via the UI. They can still do validation and run through the normal pathway. Move a couple settings to use this.
- Remove `getPreference()` on `PhabricatorUser`, which was a sort of prototype version of `getUserSetting()`.
- Make `getUserSetting()` validate setting values before returning them, to improve robustness if we change allowable…