Set promo adjustment to 0 when not actionable

If an order qualifies for a line item promotion due to meeting some condition
(say, 10% off 5 or more), and the cart is updated so that the line item
promotion no longer qualifies, this will ensure that the line item promotion
is set to zero.

There is additionally some changes to hide promotions which are zero, as
they're not super interesting. The promotions are zero-d out and hidden so
that if at some point in the future the order re-qualifies, the amount will be
set correctly.