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 couple hours for an important meeting, and returning to SF on a later flight).
• Return to SF for a few days.
• Travel back to NY.
• You should be prompted again, since you left the timezone after you ignored the discrepancy.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T3025

Differential Revision: https://secure.phabricator.com/D15961