code: protected SignupPanel<BaseUsernameSignUpModel> newSignupPanel(final String id,
  		final IModel<BaseUsernameSignUpModel> model)
  	{
  		return new SignupPanel<>(id, model);
  	}
docs: Factory method for creating the SignupPanel. This method is invoked in the constructor from
  the derived classes and can be overridden so users can provide their own version of a
  SignupPanel.
  
  @param id
  the id
  @param model
  the model
  @return the SignupPanel