code: func logoutButton(r *messenger.Response) error {
  	buttons := &[]messenger.StructuredMessageButton{
  		{
  			Type: "account_unlink",
  		},
  	}
  	return r.ButtonTemplate("Unlink your account.", buttons, messenger.ResponseType)
  }
docs: // logoutButton show to the user a button that can be used to start
  // the process of unlinking an account.