Refactor page and group user handling

Hello, Currently there's three tickets related to issues with the current implementation of our user and groups models. https://github.com/divio/django-cms/issues/3532 https://github.com/divio/django-cms/issues/3863 https://github.com/divio/django-cms/issues/5328 Basically we're relying on the

Updating documentation examples for Python 3

We received a pull request https://github.com/divio/django-cms/pull/5390, for a documentation example. The code sample uses a Python-2-style __unicode__() method; in Python 3 it would be __str__(). It’s quite right that this needs to be updated, but we’d like to do it in a more systematic way.