Events should offer Spaces as the view policy options

Summary: Ref T8687, Events should offer Spaces as the view policy options

Test Plan: Create event, choose default Space, save, edit, choose different Space, save, new policy should be reflected on Event.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T8687

Differential Revision: https://secure.phabricator.com/D13459