Make default start value of new events to be the next whole hour and the end value to be the next whole hour after that.

Summary: Ref T8031, Make default start value of new events to be the next whole hour and the end value to be the next whole hour after that.

Test Plan: Create new event at, say, 10:30am. Event default times should be 11:00 AM - 12:00 PM

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8031

Differential Revision: https://secure.phabricator.com/D12954