It's not really needed as it doesn't make sense to simply add an address
to your addressbook as part of checkout.

Requires a few fiddly changes to basket views, and a slight change to
how ui.js renders flash messages from AJAX requests.

This view has a form that assumes anon checkout is allowed, which it
isn't always.  Treating it as a special case was causing it to be
displayed when it shouldn't.

New look is a bare form for each stage as opposed to showing all the
information.

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.