Allow applying of fixed discounts per product

Right now the default behavior for fixed discounts is to apply a single discount to each product type (e.g. a 10 SEK pensioner rebate would be applied once if 4x product A are added, and once if 1x product B is added).

We would like the ability to apply fixed discounts per product (e.g. if we add 4x product A, the rebate is applied 4x).

The only viable workarounds now are to create new products for the same products with the discount (which creates many duplicate products), or to apply a % discount instead (this doesn’t come out to a clean number).