R1:edfc6a69345a

Convert some loadPreferences() to getUserSetting()

Summary:
Ref T4103. This doesn't get everything, but takes care of most of the easy stuff.

The tricky-ish bit here is that I need to move timezones, pronouns and translations to proper settings. I expect to pursue that next.

Test Plan:
- Grepped for `loadPreferences` to identify callsites.
- Changed start-of-week setting, loaded Calendar, saw correct start.
- Visited welcome page, read "Adjust Settings" point.
- Loaded Conpherence -- I changed behavior…