Proper handling of invalid/unknown time zones

Proper handling of invalid/unknown time zones

Currently, a user who has an invalid time zone stored in the database is
effectively locked out of their account on HHVM sites. This patch addresses
this by (1) preventing users from setting invalid time zones, and (2) not
throwing an unhandled exception if a user's TZ is unknown.

When the user saves their preferences, the code silently rewrites invalid
time zones to UTC. I think this is OK, since to cause this to happen you
have to manually muck around with the Preferences page DOM or submit the
form from a script.

Bug: T137182
Change-Id: I28c5e2ac9f2e681718c6080fb49b3b01e4af46dd