Explicitly enable LDAP referrals

Note: The mock LDAP system doesn't support referrals, so we can't add a unit test for this.

data/users.py

@@ -43,6 +43,7 @@ class LDAPConnection(object):
   def __enter__(self):
     trace_level = 2 if os.environ.get('LDAP_DEBUG') == '1' else 0
     self._conn = ldap.initialize(self._ldap_uri, trace_level=trace_level)
+    self._conn.set_option(ldap.OPT_REFERRALS, 1)
     self._conn.simple_bind_s(self._user_dn, self._user_pw)
 
     return self._conn