Summary: Adds ability to set visibility when authoring a Post. New default is "Visible". If you write a post and save it as a Draft, and later click publish, a feed story and mail will go out.
Test Plan: Write a new Post, see feed story and get email. Write a new Draft, get nothing. Click Publish, see story and email.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Maniphest Tasks: T9360
Differential Revision: https://secure.phabricator.com/D14429
