User's credit notes should be applied automagically to new invoices

[chrisjrn]

When a user has credit notes, they should all be applied automatically to new invoices as they are created.

[chrisjrn]

Notes:

1. If a user has multiple invoices pending (e.g. because a manual invoice was created), then this feature does not apply.
2. If a user has multiple credit notes, apply as many of them as is necessary to pay off the invoice
3. Once an invoice has zero remaining balance, stop applying further credit notes