Add timezone support

Summary:
Allows user-configurable timezones. Adds a preference panel, and migrates to the
new date rendering in easily-modified areas of the code. *In progress*.

Test Plan:
Check database to make sure the field is being changed when the settings are
changed; check affected views to see how they render times.

Reviewed By: epriestley
Reviewers: epriestley
CC: aran, epriestley, toulouse
Differential Revision: 475