Ability to invite users, but no RSVP capability yet.

Summary: Ref T7986, Ability to invite users, but not RSVP yet.

Test Plan: Open event, add invitees, save, event detail view should show invitees.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7986

Differential Revision: https://secure.phabricator.com/D12617