Allow setting a billing address on the subscription

[aldesantis]

Closes #117.

Adds the ability for users and admins to set the billing address on their subscription:

• When creating a subscription on the backend, the billing address is set directly on the subscription through the backend UI.
• When creating a subscription on the storefront or via the API, the billing address is copied from the order, just like what happens for the shipping address.

The subscription processing logic has also been adjusted slightly in order to:

• group subscription processing jobs by both shipping and billing address, and
• use either the subscription's billing address or the user's default billing address when processing a subscription.