Improve customization for forms

[rubengrill]

It is very hard to use a custom form, especially when using REST:

1. Create a custom form
2. Create a custom serializer using that form
3. Create a custom view using that serializer
4. Don't use include to include all nopassword urls, but instead include all urls seperately just to be able to use the new view

It would be great if we would support dynamic loading of the used form classes, in the same way as django rest auth is doing it:
https://github.com/Tivix/django-rest-auth/blob/master/rest_auth/app_settings.py#L28

They have a setting REST_AUTH_SERIALIZERS where custom serializer class names can be specified , and those serializers are loaded dynamically.
We could provide some similar setting NOPASSWORD_FORMS, those forms would be used both by the regular views and by the serializers.
This way providing a custom form + changing settings would be enough to integrate custom forms.