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/models.py

@@ -0,0 +1,17 @@
+from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
+from django.utils.translation import ugettext as _
+
+from model_utils.models import TimeStampedModel
+
+
+@python_2_unicode_compatible
+class MailDomain(TimeStampedModel, models.Model):
+    domain = models.CharField(max_length=128, unique=True)
+
+    class Meta:
+        verbose_name = _('Mail domain')
+        verbose_name_plural = _('Mail domains')
+
+    def __str__(self):
+        return self.domain