[JRO] Add `girdler` for incoming email

Incoming email is a necessary feature. This commit allows
the engine to pull in posts and topics sent as emails.
[JRO] Clean-up on aisle 5!

* Ignore uploads completely in the dummy app
* White-space
* Remove `admin_link_for`. It's not used.
* Specify the only two actions in PreferencesController
[JRO] Refactor users browsing history

* user_topic_decorator had been running a query against the
  topic.user_topic_reads association. The topic object has 
  eager loaded that relationship already so there is no need 
  to run another query against it. Using select instead of a 
  `where` removes the need to make the query.
* Clean up posts controller
* Return a bare post object in null_topic_read#farthest_post
  instead of nil
* Ensure uniqueness of `farthest_post` and `user_id` columns 
  on user_topic_read