Ability to join or decline an event

Summary: Ref T7986, Ability to join or decline an event.

Test Plan: Open Calendar event, join event, Invitee list should update. Decline event, invitee list should remove you.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7986

Differential Revision: https://secure.phabricator.com/D12618