define User.is_sftp_user and fix minor template issues

gnuviechadmin/osusers/models.py

@@ -266,6 +266,10 @@ class User(TimeStampedModel, models.Model):
             _LOGGER.info("set LDAP password for %s", dn)
             return True
 
+    def is_sftp_user(self):
+        return self.additionalgroup_set.filter(
+            group__groupname=settings.OSUSER_SFTP_GROUP
+        ).exists()
 
     @transaction.atomic
     def save(self, *args, **kwargs):