introduce new app domains

- generate domains app
- duplicate managemails.models.MailDomain as domains.models.MailDomain
- add domains app to gnuviechadmin.settings.base.LOCAL_APPS
- repair managemails.tests.test_models
- fix handling of empty results in osusers.models.GroupManager.get_next_gid and
  osusers.models.UserManager.get_next_uid

gnuviechadmin/domains/tests/test_models.py

@@ -0,0 +1,9 @@
+from django.test import TestCase
+
+from domains.models import MailDomain
+
+
+class MailDomainTest(TestCase):
+    def test__str__(self):
+        md = MailDomain.objects.create(domain='example.org')
+        self.assertEqual(str(md), 'example.org')