Ability to RSVP to event

Summary: Closes T7986, Ability to RSVP to event.

Test Plan: Create event, invite users, invited users should see buttons on event detail header to accept/decline. Accepting/declining reflects in the invitee status in the property list.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7986

Differential Revision: https://secure.phabricator.com/D12619