code: func (h *Handler) u2fRegisterRequest(w http.ResponseWriter, r *http.Request, p httprouter.Params) (interface{}, error) {
  	token := p[0].Value
  	u2fRegisterRequest, err := h.auth.GetUserInviteU2FRegisterRequest(token)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  
  	return u2fRegisterRequest, nil
  }
docs: // u2fRegisterRequest is called to get a U2F challenge for registering a U2F key
  //
  // GET /webapi/u2f/signuptokens/:token
  //
  // Response:
  //
  // {"version":"U2F_V2","challenge":"randombase64string","appId":"https://mycorp.com:3080"}
  //