Add private room capability

A "challenge" can be made private by checking a box in the Django admin.  This transforms the challenge into "private room" mode.

* The private room is accessed via the ".../room/slug/" URL, rather than the standard ".../challenge/id/" format
  - Trying to access a private room using the '../challenge/id' format will throw an error
  - This was done to prevent users from snooping around by changing the id number in the URL
* There are no access controls yet, everyone has access to all private rooms (if they know the URL)
* Ideas submitted to a private room are not accessible from the public view
* Tags for ideas in a private room are not linked/counted in the public space, and vice versa
* "Submit Idea" and "Back To..." buttons direct to the private room home page, rather than the IdeaBox homepage
  - This was done to prevent people from accidentally bouncing out of the private page's scope.