Allow to use not used discounts on other items

* When making a sale, some items may not be using all of it's discount. This
  makes it possible for other items to used that discounted on themselves.
* When making a sale quote, add an extra button that will allow us to apply a
  discount to all items at one time

Change-Id: I1b3513982ea749184eaf21deeefa63f2dd1d4997