Validate timezones

Summary:
Add validation for timezones, since date_default_timezone_set() returns a usable error code.

Note that we could also list all the timezones using timezone_identifiers_list(), but the list is enormous (many hundreds of entries) and impossible to use (~160 entries in "America" alone). I listed the likely US values as examples but left it as a string input text field.

Test Plan: Tried to save an invalid setting. Saved a valid setting.

Reviewers: codeblock, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4318