When a user changes their timezone, clear their ignored timezone offset

Summary:
Ref T4103. We have a couple of settings like this where changing one setting changes another (e.g., enabling DarkConsole makes the console visible).

Provide a mechanism to let changing timezone really mean "change timezone, and also clear the timezone offset".

Test Plan: Swapped timezones, reconciled them by ignoring the offset, changed timezone again to another zone with the same offset, got asked to reconcile again.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16018