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.

Conflicts:
	frontend/app/views/spree/orders/_form.html.erb