Don't lock subscription in PhameBlog

Summary: Ref T11687. Subscription to Blogs comes with many additional features, don't lock people in.

Test Plan: Saw I was no longer subscribed.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11687

Differential Revision: https://secure.phabricator.com/D16589