Add BTW to products

[JulesFleuren]

It would be nice if the BTW tariff could be specified for products. It should be possible to choose from three different percentages: 0%, 9% and 21%.
Then the mail containing the tickets can specify the BTW amount and be used as a receipt for the participants.
To calculate the BTW amount, take the percentage of the total amount and round off per product.

[JoepdeJong]

I suggest to add a tax field to the products which can be selected from the following list.

• TAX_FREE No tax at all
• TAX_ZERO 0% tax
• TAX_LOW 9% tax
• TAX_HIGH 21% tax

If we implement this as an enum, we can later change all the tax rates. We can use TAX_FREE as default when implementing this.

@JulesFleuren Do you, as a treasurer, use events as a history for orders? I don't know how often the tax rates update, but if necessary, we have to find some way to store all tax values on older orders. Or is this not a problem?