Don't crash with invalid timezones.
Don't crash with invalid timezones.

If a user's browser is sending an invalid timezone string, we shouldn't prevent
them from using Review Board.

Fixes bug 3594.

Reviewed at https://reviews.reviewboard.org/r/6501/

KnowNo/reviewboard,chipx86/reviewboard,davidt/reviewboard,sgallagher/reviewboard,KnowNo/reviewboard,bkochendorfer/reviewboard,sgallagher/reviewboard,bkochendorfer/reviewboard,chipx86/reviewboard,custode/reviewboard,sgallagher/reviewboard,custode/reviewboard,brennie/reviewboard,reviewboard/reviewboard,beol/reviewboard,beol/reviewboard,bkochendorfer/reviewboard,brennie/reviewboard,custode/reviewboard,brennie/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,bkochendorfer/reviewboard,davidt/reviewboard,custode/reviewboard,davidt/reviewboard,reviewboard/reviewboard,beol/reviewboard,davidt/reviewboard,KnowNo/reviewboard,reviewboard/reviewboard,beol/reviewboard,brennie/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,KnowNo/reviewboard