Implement PhabricatorDestructibleInterface in Phame

Summary: Allows Blogs and Posts to be destroyed. Fixes T9756

Test Plan: Test bin/remove destroy POST and bin/remove destroy BLOG to great success.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9756

Differential Revision: https://secure.phabricator.com/D14586