Page permissions aren't working anymore

[timobrembeck]

Describe the Bug

Since our change to a custom user model, our page object-specific permissions aren't working anymore.
Apparently, django-rules has problems with this (see dfunckt/django-rules#151).
The biggest problem is that it breaks the normal page permissions, even if page-specific permissions are disabled.

Steps to Reproduce

1. Create a non-superuser which should have the permissions to edit all pages
2. Login with this user and open a page form

Expected Behavior

The user should be able to edit the page

Actual Behavior

It's not possible to edit/publish the page and the following messages are shown:

Additional Information