The previous skip condition for shipping bypassed the shipping method
view but this is required as it sets the shipping method code.

When ordering a basket of free products, or with vouchers giving 100%
discount, we should not collect payment details.

To achieve this, I've added a check to the CheckoutSessionMixin which
calculates the order total and decides whether payment is necessary.

The modified test failed because the OrderCalculator called by
check_payment_is_necessary accessed other properties on the shipping
method. We now just mock a valid shipping method, which still achieves
the aim of ensuring there's only one.

A bit OCD, but I noticed that the checkout forms were an exception in
using a hyphen so I've cleaned up all IDs to use underscores.

We no longer use Django's test client.

- Comment the session manager better.
- Clean up the functional checkout tests.

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.