Adding account vat on payment 7

Moved from https://code.launchpad.net/~domsense/account-payment/adding_account_vat_on_payment_7/+merge/196914
This could be simpler with like this:
```python
allocated_amount = amounts_by_invoice[invoice.id]['allocated']
if not (
    voucher.exclude_write_off
    and voucher.payment_option == 'with_writeoff'
):
    allocated_amount += amounts_by_invoice[invoice.id]['write-off']
```