Provide phame.post.edit Conduit API method

Summary:
Ref T9897. This one is a little more involved because of how getting a post on a blog works.

I also changed moving posts to be a real transaction (which shows up in history, now).

Test Plan: Created posts from web UI and conduit.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9897

Differential Revision: https://secure.phabricator.com/D14902