Clicking in day view should create new event

Summary: Ref T8300, clicking in day view should create new event

Test Plan: Open day view, click in an empty slot, new event modal should open.

Reviewers: epriestley, #blessed_reviewers, chad

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8300

Differential Revision: https://secure.phabricator.com/D12990