"Remove related objects for these users" (admin interface action) fails with 500 error

[jnm]

This custom Django admin interface action allows us to delete user accounts by:

1. Removing all their related KPI objects (from KPI's admin interface);
2. Removing all their related KC objects and their account itself (from KC's admin interface).

This must be fixed to complete https://github.com/kobotoolbox/tasks/issues/32.

django-guardian (within KPI) is choking on the KoBoCAT permissions: https://sentry.kbtdev.org/kobo/kpi-backend/issues/18761/

AttributeError: 'NoneType' object has no attribute '_base_manager'
  File "django/core/handlers/base.py", line 132, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django/contrib/admin/options.py", line 618, in wrapper
    return self.admin_site.admin_view(view)(*args, **kwargs)
  File "django/utils/decorators.py", line 110, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "django/views/decorators/cache.py", line 57, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "django/contrib/admin/sites.py", line 233, in inner
    return view(request, *args, **kwargs)
  File "django/utils/decorators.py", line 34, in _wrapper
    return bound_func(*args, **kwargs)
  File "django/utils/decorators.py", line 110, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "django/utils/decorators.py", line 30, in bound_func
    return func.__get__(self, type(self))(*args2, **kwargs2)
  File "django/contrib/admin/options.py", line 1576, in changelist_view
    response = self.response_action(request, queryset=cl.get_queryset(request))
  File "django/contrib/admin/options.py", line 1335, in response_action
    response = func(self, request, queryset)
  File "hub/actions.py", line 57, in delete_related_objects
    modeladmin.admin_site, using
  File "django/contrib/admin/utils.py", line 153, in get_deleted_objects
    to_delete = collector.nested(format_callback)
  File "django/contrib/admin/utils.py", line 213, in nested
    roots.extend(self._nested(root, seen, format_callback))
  File "django/contrib/admin/utils.py", line 198, in _nested
    ret = [format_callback(obj)]
  File "django/contrib/admin/utils.py", line 126, in format_callback
    force_text(obj))
  File "django/utils/encoding.py", line 92, in force_text
    s = six.text_type(s)
  File "guardian/models.py", line 35, in __unicode__
    unicode(self.content_object),
  File "django/contrib/contenttypes/fields.py", line 235, in __get__
    rel_obj = ct.get_object_for_this_type(pk=getattr(instance, self.fk_field))
  File "django/contrib/contenttypes/models.py", line 196, in get_object_for_this_type
    return self.model_class()._base_manager.using(self._state.db).get(**kwargs)

[jnm]

lolll thanks past self:

kpi/kpi/deployment_backends/kc_access/utils.py

Lines 294 to 296 in 0b09a60

	# Do NOT try to `print` or do anything else that would `repr()` this
	# queryset, or you'll be greeted by
	# `AttributeError: 'NoneType' object has no attribute '_base_manager'`