CalendarEvent implements PhabricatorTokenReceiverInterface

Summary: Closes T7946, CalenderEvent implement PhabricatorTokenReceiverInterface.

Test Plan: Create or open a calendar event, award a token, rescind the token, see the full timeline under the event details, open Feed, only token award action should have surfaced.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7946

Differential Revision: https://secure.phabricator.com/D12600