core: add custom group model with hierarchy , add tree admin

passbook/admin/api/v1/urls.py

@@ -0,0 +1,9 @@
+"""passbook admin API URLs"""
+from rest_framework.routers import DefaultRouter
+
+from passbook.admin.api.v1.groups import GroupViewSet
+
+router = DefaultRouter()
+router.register(r'groups', GroupViewSet)
+
+urlpatterns = router.urls