When we encounter a fanciful timezone identifier, try to guess what it might mean
Ref T11816. My read of RFC 5545 is that applications can do whatever they want here. Although most applications use legal timezonedb values like "America/Los_Angeles", at least one ("Zimbra") has at least one event with a weird value (TZID="(GMT-05.00) Auto-Detected").
Try to puzzle out what these mysterious identifiers might intend. For now, I added a rule to look for "UTC+3", "GMT-2:30", etc.
If we don't have any guesses, just use UTC. If we guess or fall back, raise a warning so the user can see what happened.
Test Plan: Added a unit test. See also next change.
Reviewed By: chad
Maniphest Tasks: T11816
Differential Revision: https://secure.phabricator.com/D16800