use GetResponseUserEvent on registration init

The [ChangePasswordController](https://github.com/FriendsOfSymfony/FOSUserBundle/blob/e060f8c51abefbb1e0fce2fb18de45474c8a3504/Controller/ChangePasswordController.php#L45-L50) uses the `GetResponseUserEvent` to allow custom responses when initializing change password process; just like the other controllers.

However the [RegistrationController](https://github.com/FriendsOfSymfony/FOSUserBundle/blob/e060f8c51abefbb1e0fce2fb18de45474c8a3504/Controller/RegistrationController.php#L46) isn't utilizing this type of event right now, is there any special reason behind this?