[IMP] point_of_sale: enable product re-ordering in the point of sale.
[IMP] pos_loyalty: make sure that the gift products are available in the point of sale
[IMP] pos_loyalty: discount loyalties are now history based.
- The discount percentage applies to the total sale amount
  of the customer on the previous orders.
- Once the discount is applied, the history used to compute
  the discount is discarded.
- A customer's history can be seen in the partner's Point of
  Sale tab.