Setting to show notification option on registration form or not

The patch attached add an option in admin setting form of notify,
which allows administrator to select whether to display the option to enable notification on user registration form or not.
It also has an option to set the default value to be displayed on registration form.

The patch was created for my client's website,
where we want to use notify to send emails to administrators if new content is created on website,
but ordinary users are not allowed to use notify.
So it would be quite odd to have the option for enabling notification on user registration form,
which would be available to public.