Make timezone configuration impossible to get wrong

Summary: Fixes T2269. If the user manages to mess up both the PHP and Phabricator configurations, set the timezone to UTC. We basically never use this anyway (we always render into the user's time), PHP just gets angry at us if we don't set it. (We do use it for logged-out users, I suppose.)

Test Plan: Set PHP and Phabricator timezones to goofy nonsense, verified we recover sensibly from it.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2228, T2269

Differential Revision: https://secure.phabricator.com/D4496