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

passbook/admin/views/groups.py

@@ -0,0 +1,12 @@
+"""passbook Group administration"""
+from django.views.generic import ListView
+
+from passbook.admin.mixins import AdminRequiredMixin
+from passbook.core.models import Group
+
+
+class GroupListView(AdminRequiredMixin, ListView):
+    """Show list of all invitations"""
+
+    model = Group
+    template_name = 'administration/groups/list.html'