Explicitly pass the billing address around checkout

This changeset adjusts `build_submission` to include the billing address.
This also requires changes to a few method signatures to handle the
billing address explicitly.

This only works when the billing address is stored in the session (as
that's where it is looked up from when building the submission data in
`build_submission`). In many cases (such as Oscar's demo site), we don't
store the billing address in the session - instead, it is captured via
the payment-details forms. Hence, the billing address will still need to
be explicitly set within the submission data.

Fixes #1508
Merge pull request #1552 from django-oscar/issue/1508

Explicitly pass the billing address around checkout