message: Store retrieved sid in the signupInstance of EmailIdentityStage
  When registeration is complete, the RTS needs the sid, which was previously only sent to the HS. This update will also store it in the signupInstance so that it can be sent to the RTS.
diff: @@ -149,6 +149,7 @@ class EmailIdentityStage extends Stage {
  nextLink
  ).then(function(response) {
  self.sid = response.sid;
  + self.signupInstance.setIdSid(self.sid);
  return self._completeVerify();
  }).then(function(request) {
  request.poll_for_success = true;
  