RoutablePage

This class has been useful on a couple of occasions.

This class was created a couple of months ago to help with embedding url confs into pages. Its not been added into wagtail because there isn't really a place for it to go. But since #249 introduces a contrib folder I thought this would be a good place for this class to live.

Let me know what you think of this idea

TODO:
 - [x] Wait for frontend permissions to be merged (to be sure that this will work with the new routing)
 - [x] Tests
 - [x] Documentation