Calendar events should support subscribers, for possible future uses.
Summary: Closes T7974, Calendar events support subscribers.
Test Plan: Create or update calendar event, add subscribers, save, event detail view should show subscribers, and timeline should reflect the action.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin, epriestley
Maniphest Tasks: T7974
Differential Revision: https://secure.phabricator.com/D12595