Improve handling of group types for school structure

aleksis/apps/cursus/util/group_types.py

+from aleksis.core.models import GroupType
+
+from ..settings import SCHOOL_CLASS_GROUP_TYPE_NAME, SCHOOL_GRADE_GROUP_TYPE_NAME
+
+
+def get_school_grade_group_type():
+    group_type, __ = GroupType.objects.managed_by_app("cursus").get_or_create(
+        name=SCHOOL_GRADE_GROUP_TYPE_NAME, managed_by_app_label="cursus"
+    )
+    return group_type
+
+
+def get_school_class_group_type():
+    group_type, __ = GroupType.objects.managed_by_app("cursus").get_or_create(
+        name=SCHOOL_CLASS_GROUP_TYPE_NAME, managed_by_app_label="cursus"
+    )
+    return group_type