Use correct lookup field on RelatedProjectQuerySetBase

`self` is either ChildRelatedProjectQuerySet or
ParentRelatedProjectQuerySet. None of these one has a `project`
field. Because of this, we need to use `self.project_field` for lookup.

Author: humitos

readthedocs/projects/querysets.py

@@ -137,7 +137,8 @@ def _add_user_repos(self, queryset, user=None):
             return self.all()
         if user.is_authenticated:
             projects_pk = user.projects.all().values_list('pk', flat=True)
-            user_queryset = self.filter(project__in=projects_pk)
+            kwargs = {'%s__in' % self.project_field: projects_pk}
+            user_queryset = self.filter(**kwargs)
             queryset = user_queryset | queryset
         return queryset