Rescheduling events by dragging them in day view

Summary: Ref T8300, Rescheduling events by dragging them in day view

Test Plan: Open day view, drag events, observe them reschedule.

Reviewers: chad, epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8300

Differential Revision: https://secure.phabricator.com/D12988