Show more of UTC offset when user's TZ is not an integer number of hours offset

Summary: See https://discourse.phabricator-community.org/t/personal-timezone-setting-mismatch-cleared-and-more-specific-cases/1680. The code has always worked correctly, but the resulting timezone mismatch warning messsage wasn't specific enough when the mismatch is by a non-integer number of hours.

Test Plan: Set timezone locally to Asia/Vladivostok and in Phabricator to Australia/Adelaide (which as of today's date are 30 minutes apart) and observed a more precise error message: F6061330

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19873