[FIX] point_of_sale,pos_loyalty: count combo as one product for discount

[robinengels]

When computing the cheapest line of an order to apply a discount on it, combo line should be considered as one product as it is done in the sale app.

Steps to reproduce:

• Setup a combo product
• Setup a discount program that applies on the cheapest line
• Open a PoS and add the combo product

Observation: The discount would be applied on the cheapest product of the combo instead of the whole combo

Why the fix:

When we look for the cheapest line and we encounter a combo product we sum all the lines of the combo to compute it's total price

opw-4033960

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[robodoo]