Create button should be a dropdown with public and private options

Summary: Ref T8026, Create button should be a dropdown with public and private options

Test Plan: Create both a public and private event. Confirm view policies apply correctly.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8026

Differential Revision: https://secure.phabricator.com/D13033