Pull#1604 for 2.4 (develop branch) - Fix admin slug bug

As requested in https://github.com/divio/django-cms/pull/1604

Please note, that the curious code block causing the "slug bug" in `cms.admin.forms.PageAddForm.clean` has only been fixed and not removed, since I'm not into its function: -> https://github.com/divio/django-cms/blob/develop/cms/admin/forms.py#L116