text: Title: Setting to disable new user registration (the `/register` endpoint)
  Body: ### Checklist
  
  *   [x] I read [Contribution Guidelines](https://github.com/apragacz/django-rest-registration/blob/master/CONTRIBUTING.md#issues)
  *   [x] I searched [the documentation](https://django-rest-registration.readthedocs.io/) to ensure that the requested feature is not already implemented and described
  *   [x] I searched existing issues before opening this one
  
  ### Is your feature request related to a problem? Please describe.
  Sometimes, we need to stop new users from registering, while still continuing to provide the rest of the functionalities from this library to existing users.
  
  ### Describe the solution you'd like
  I would like to be able to disable registration of new users (the `/register` endpoint) based on a [setting](https://django-rest-registration.readthedocs.io/en/latest/detailed_configuration/all_settings.html).
  
  ### Describe alternatives you've considered
  Reading through the list of existing settings (and reading the code to make sure there isn't an undocumented setting), there doesn't seem to be any :upside_down_face:
  The best I can come up with (but haven't had time to test yet) is to define a custom [`REGISTER_SERIALIZER_CLASS`](https://django-rest-registration.readthedocs.io/en/latest/detailed_configuration/all_settings.html#register-serializer-class) that would be a copy of [the one already provided](https://github.com/apragacz/django-rest-registration/blob/master/rest_registration/api/serializers.py#L96) with an added exception at the beginning of `validate()`, thrown based on a django setting.