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.

Also added tests for vouchers

Also split them into unit and functional tests.

This allows the preview to accept two types of POST:

1) POSTing from payment-details to submit sensitive data, which gets
   re-rendered in the preview template
2) POSTing from preview to submit the orderk

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

* The views check that the guest email address has been entered.
* The email address is written out against the order.
* An email confirmation is sent to the guest user email address.

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.

tests