Terminology:

- A *pre*-condition is a condition that must be met in order for a view
  to be available. If it isn't then the customer will be redirected to
  a view *earlier* in the chain.

- A *skip*-condition is a condition that must NOT be met in order for a
  view to be available. If the condition is met, this means the view can
  be skipped and the customer should be redirected to a view *later* in
  the chain.

This change implements the above by introducing skip conditions for
checkout views. They allow the previous payment pre-conditions to be
cleanly separated.

An existing basket pre-condition is also changed to be a skip-condition.

It is now replaced with a checkout pre-condition that applies to every
checkout view. If someone's basket becomes invalid while they are in
checkout, then they will be redirected back to the basket page.

This change introduces a new framework for enforcing checkout
pre-conditions. Each view class can define a class attribute
'pre_conditions' which lists method names to run before the class
itself.

Of course, decorators are more conventional for this kind of thing but
they are harder to override and customise.