Tidy imports in customer views
Ensure form error messages are red
Refactor how auth forms are bound

This addresses a comment that the way requests are passed around is
confusing:
https://github.com/tangentlabs/django-oscar/commit/032a300ab61e7892108a7ec64a7304c3e03a4363#commitcomment-5482379

The change alters how the login and registration forms are constructed
to make it more explicit when we want one form to be bound.