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/managemails/tests/test_models.py

@@ -1,6 +1,7 @@
 from django.test import TestCase
 from passlib.hash import sha512_crypt
 
+from osusers.models import User
 from managemails.models import (
     MailAddress,
     MailDomain,
@@ -16,7 +17,8 @@ class MailDomainTest(TestCase):
 
 class MailboxTest(TestCase):
     def test_set_password(self):
-        mb = Mailbox.objects.create(username='test', uid=1000, gid=1000)
+        user = User.objects.create_user()
+        mb = Mailbox.objects.create(username='test', osuser=user)
         mb.set_password('test')
         self.assertTrue(sha512_crypt.verify('test', mb.password))