[FIX] Convert tax tags based on move type instead of amount.

[ddejong-therp]

Like the title says.

The issue is that tax tags shouldn't be based on whether a line is of positive or negative amount, but e.g. whether it is an 'invoice' or 'refund'.

[pedrobaeza]

You may have tax tags on other moves different from invoices (for example, payroll moves), so I don't think this is correct.