Fix FIXMEs from invoice.py

[wdoekes]

        for ledger_line in self.get_ledger_lines():
            # We must use VATCode. It accepts VATPercentages, but only
            # when it is higher than 0. Not using:
            # 'VATPercentage': str(ledger_line['vat_percentage'] / 100)
            if ledger_line['vat_percentage'] == 0:
                vatcode = '0  '  # FIXME: hardcoded.. fetch from API?
            elif ledger_line['vat_percentage'] == 21:
                vatcode = '2  '  # FIXME: hardcoded.. fetch from API?