Add a transaction for PhamePost visibility

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