Use a generic reference to Django User model.

Use django.contrib.auth.get_user_model() to reference the django User. This way it is possible to use a custom User model.
https://docs.djangoproject.com/en/dev/topics/auth/customizing/#referencing-the-user-model
Seeing as `first_name`, `last_name`, and `email` are already on the `MangoPayLegalUser` model. I think instead of adding the fields again to the `MangoPayNaturalUser` model they should be moved to the `MangoPayUser` model along with the properties.