Fix several issues with the CheckoutSessionMixin example given for
using a DeferredTax tax assessment approach:

- Test for submission['shipping_address'] and
  submission['shipping_method'].
- Update line item tax via line.purchase_info.price.tax.
- Refer to shipping_charge rather than shipping_method where
  appropriate.
- Base the shipping tax rate on the tax-exclusive, not tax-inclusive
  shipping charge.
- Fix indentation for the calculate_tax convenience function.

Fixes issue django-oscar/django-oscar#1966.

This might be resurrected in a separate repository in the future.

- Rework the page on applying taxes in the US
- Update the release notes
- Update the "sandbox" page with details of the new site

Fixes #1266