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 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().

Test Plan:

• Changed search scopes.
• Reconciled local and server timezone settings by ignoring and changing timezones.
• Changed date/time settings, browsed Calendar, queried date ranges.
• Verified editor links generate properly in Diffusion.
• Browsed around with time/date settings looking at timestamps.
• Grepped for getPreference(), nuked all the ones coming off $user or $viewer that I could find.
• Changed accessiblity to high-contrast colors.
• Ran all unit tests.
• Grepped for removed constants.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16015