Calendar events should now surface to Feed.

Summary: Closes T7955, Calendar events should now surface to Feed.

Test Plan: Create and/or edit an calendar event, open Feed, inspect the Feed stories for the event.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7955

Differential Revision: https://secure.phabricator.com/D12597