Tumblr posts should be deleted when users delete their originating Diaspora post.

This implements cascading deletion for Tumblr posts created through Diaspora's Tumblr service. (I realize there's no spec file attached; I don't know much about them, [and SOMEONE is encouraging me to send a pull request early](https://github.com/diaspora/diaspora/issues/3938#issuecomment-21774273)….) ;)