Fixed #6327 -- Added has_module_permission method to BaseModelAdmin

* Added tests for this new method and for the other ones:
  * has_add_permission()
  * has_change_permission()
  * has_delete_permission()
* Updated the documentation
I wasn't sure about that, but if you override has_module_permission() to block acces to a certain module, someone with add, change, or delete permission would still be able to access those pages by entering the correct url.