Added Fixed Price Cart Discounts

We added support for fixed price discounts on line items with an improved Cart Discount creation. With this new discount type, you can model cart promotions like these:

• Final Sale - all articles of the 'Sales' category in the cart for 15€ each.
• Buy a pair of sunglasses for 10€ with coupon code SUNGLASSES (not limited to one pair, all pairs will become 10€).
• Phone accessories are only $10 with the purchase of any new phone (not limited to one accessory, all accessories will become $10).
• Get any 2 shirts for $50 each (not limited to two shirts, all shirts will become $50 if there are at least 2 shirts in the cart).