More tax removal options

[lukeholder]

When skipping tax application due to the presence of a valid tax ID, set the rate to zero instead so that the rate is still shown on the cart.

We also need to look into not using a discount adjustment to remove the included tax.

[joostwaaijer]

In my issue #4208 related to this discussion, we have a client that has a webshop with mixed customers, normal customers and businesses.

For us it's natural to fill prices including taxes and then, when someone has a valid tax ID, the tax amount gets removed. Currently the removal of tax is seen as a "discount". When also adding a coupon code to the cart, you'll see the totalDiscount as "tax removal" + "discount", which is not how it should be. You still want to be able to separate the Tax and Discount adjustments.

If tax can be separated from "discount" we can show positive or negative values for Tax, based on how product prices are added.

• If product prices include taxes, you can show a negative amount when there is a valid tax ID
• If product prices don't include taxes, you can show a positive amount when there is no valid tax ID

I feel like the "discount" rules could also have some extra options for their behavior, so the discount gets applied after removing or adding taxes, and using the price after adjustments.

In our case, we would like customers to receive discount over the "taxed product price" and business customers to receive discount over the "taxed product price - tax".