Ignore Calendar date edits which just change the internal date timezone without rescheduling it

Summary:
Ref T11816. Currently, if someone in California creates an event and then someone in New York edits it, we generate a no-op "<user> changed the start time from 3PM to 3PM." transaction.

This is because the internal timezone of the event is changing, but the actual absolute time is not.

Instead, when an edit wouldn't reschedule an event and would only change the internal timezone, ignore the edit.

Test Plan:

• Edited non-all-day events in PST / EST with out making changes (ignored).
• Edited non-all-day events in PST / EST with changes (changes worked).
• Performed the same edits with all-day events, which also were ignored and worked, respectively.
• Pulled events in and out of all-day mode in different timezones, behavior seemeed reasonable.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816

Differential Revision: https://secure.phabricator.com/D16955