Users should be able to rsvp to event via email

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