Respect 12 hour vs 24 hour time formats in Calendar day views

Summary: Fixes T9202.

Test Plan:

• Viewed day in 12-hour, saw "8:00 PM".
• Viewed day in 24-hour, saw "16:00".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9202, T10932

Differential Revision: https://secure.phabricator.com/D16290