Calendar events should now auto-invite the creator

Summary: Closes T7935, Calendar events should now auto-invite the creator.

Test Plan: Create event, save, event should now show creator as an invitee.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7935

Differential Revision: https://secure.phabricator.com/D12613