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 values later.
- Add a user setting cache, since reading user settings was getting fairly expensive on Calendar.
- Improve performance of setting validation for timezone setting (don't require building/computing all timezone offsets).
- Since we have the cache anyway, make the timezone override a little more general in its approach.
- Move editor stuff to use getUserSetting().