Since there is no demo or us site anymore we can simplify the structure
somewhat by moving the sandbox one directory up

 - Set Meta.app_label on models created in the tests
 - Use logging.NullHandler instead of django.utils.log.NullHandler
 - Use oscar.core.loading.get_model() instead of django's get_model
 - Remove template tags '{% load url from future %}'

Need to explicitly define a name.

Deferred benefits could reasonably make use of that information.

It assumed there was a basket user.

This alters the benefit model to allow proxy class to be registered.
The dashboard flow is tweaked to allow such classes to be selected once
they've been registered.

A dummy custom benefit is included in the sandbox.