Calendar events should be flaggable

Summary: Closes T7947, Calendar events should be flaggable.

Test Plan: Flag a Calendar event, open Flags, see flag.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7947

Differential Revision: https://secure.phabricator.com/D12602