Set event policies like other event properties

Summary: Closes T8028, Set event policies like other event properties

Test Plan: Create and edit events, make sure edit and view policies get populated and saved correctly.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T8028

Differential Revision: https://secure.phabricator.com/D13415