Summary: Closes T7957, Users should be able to rsvp to event via email.
Test Plan: Create event, open invitee email, reply to email w/ !rsvp yes, refresh event, invitee should show as attending.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin, epriestley
Maniphest Tasks: T7957
Differential Revision: https://secure.phabricator.com/D12651
