[Reverted] Make the order-order_item relationship more robust

Let's do the same thing we did for products: #3002474: Make the product-variation relationship more robust

When saving a product we ensure that all referenced variations have the product_id set. However, we don't do the opposite, when saving a variation we don't check that the specified product references the variation. There's no cost to doing so, cause we skip the save if there's nothing to do. We already do this two-way check on promotions and coupons.

This will allow us to create an order item and have it automatically be referenced by an order.