This is used by the weight-based shipping method but can be used by
others too.

This uses a few new methods in oscar.core.application.Application which
enable custom decorators to be set for given view fns.

The main use case at the moment is to make it easy to ensure that
checkout is not available to non-logged-in users when
OSCAR_ALLOW_ANON_CHECKOUT is set to False.

Also reworked the models for each promotion type.

tests

Uses a different template and redirect for logged-in users

Uses a class-based reimplementation of Django's existing function-based login view. The view also handles registration at the same URL endpoint. It depends on the submit button having a name of "login" or "registration" to determine which form was POSTed.

Auth is done using email addresses rather than username (which is a randomly generated string). The registration process enforces uniqueness, but this won't be enforced anywhere else, so we need to be careful not to duplicate email addresses in the admin or something.

Move into product app
Rewrote views to use generic classes
Tidied up POST actions of forms