Allow usage of coupons as order markers

Add "Remove When Not Effective" checkbox (ON by default) to coupons. This way we keep backwards compatibility and allow users to create coupons that will stay applied even if they don't provide any real discount just to group order by having same coupon. This way store owner can do some additional logic on his side.

Source of discount in this case (when we keep coupon even if it isn't effective) will be the same as before: discount OR none if no discount is in effect.