R1:a91004ef1ba3

Detect timezone discrepancies and prompt users to reconcile them

Summary: Ref T3025. This adds a check for different client/server timezone offsets and gives users an option to fix them or ignore them.

Test Plan:
- Fiddled with timezone in Settings and System Preferences.
- Got appropriate prompts and behavior after simulating various trips to and from exotic locales.

In particular, this slightly tricky case seems to work correctly:

- Travel to NY.
- Ignore discrepancy (you're only there for a…