[JRO] Make sure posts inherit messageboard filter

Somehow this was a regression
[WIP] Start splitting apart thredded_topics

* Create a thredded_private_topics table
* Change Thredded::PrivateTopic to subclass from AR::Base instead
* Start filling in the blanks on the dependencies - posts, decorators, etc
* Add associations, copy over code from Thredded::Topic to
  Thredded::PrivateTopic