Force order creation even when product quantity isn't enough

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Expected behaviour

When an order has been placed in a marketplace, it should be created in PrestaShop even if there is no stock available.

This is due to the fact that an order placed in a marketplace is already paid and should then be considered created. If there is issue with the availability of the product, it should be managed case by case by the the merchant or cancelled.

When there is not enough stock for a product, we should check the configuration "out of stock" for the product.

• If the product is configured to allow the creation of the order when out of stock, then we process and the final stock will be negative.
• If the product if configured to deny the creation of the order when out of stock, then we update the stock of the product accordingly to have enough quantity to allow the creation of the order from the MKP.

Actual behaviour

When there is not enough stock for a product, we are not checking the configuration "out of stock".

• Since we introduced #383, when the advanced stock management is being used, we do nothing specific (and the order will then be created or not depending of the out of stock configuration, the order may then fail).
• When the advanced stock management is not being used, if the product doesn't have enough stock we change the stock accordingly without checking the out of stock rules. Therefore we have cases where the stock was negative before the order came through (eg: -15) and then the stock go back to 0 when the MKP order is created by ShoppingFlux.

Steps to reproduce the behaviour

Create different order with different set of rules for "out of stock" and with different stock in order to reproduce the "actual behavior".

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What is required here is to comply to the expected behavior, which might however be an issue for advanced stock management where the stock is not updated in the same way that it is for classic stock.

Advanced stock rely on warehouse stock and stock movement. So we will need to take that in consideration.

As well ze need to find a way to inform the customer is the stock of a product has been "forced" by ShoppingFlux.

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406 will be elaborated during changes in progress in #349