Calendar event datepicker should auto-complete end time to be an hour from start time unless end time has been edited.

Summary: Ref T8031, Calendar event datepicker should auto-complete end time to be an hour from start time unless end time has been edited

Test Plan: Create event, edit start time, end time should adjust to an hour from start time.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8031

Differential Revision: https://secure.phabricator.com/D12960