Suggest better start/end datetimes for Calendar events

Summary:
Fixes T11638.

• Fix a regression: I broke this "round to the nearest hour" code a while ago while fiddling with datetimes.
• Improve a beahvior: from the day view, make the menu-bar "Create Event" button default to creating an event on the day you were viewing.

Test Plan: Created events from month and day views, got nice round numbers and proper day suggestions.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11638

Differential Revision: https://secure.phabricator.com/D16754