[JRO] PrivateTopic fixes

* Order correctly on PrivateTopicsController#index
* If a slug doesn't exist in a topic, use the id instead
* clean up
[JRO] Allow configuring of user path

Not everyone uses "/users/<username>" for their user path. This allows
anyone using this board to configure where they want to send people to.

* Update all references to "/users/#{user.to_s}" (etc) to use this 
  new config.