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