Introduce promotion priorities

[duxonline]

https://support.bigcommerce.com/s/article/Automatic-Promotions?language=en_US#manage-promotion-priorities

This matters once we change the following function to use subTotal instead of itemsTotal:

    let runMinRule rule quantity itemsTotal =
        match rule with
        | MinRule.NoRestriction -> true
        | ItemQuantity itemQuantity -> quantity >= itemQuantity
        | PurchaseAmount purchaseAmount -> itemsTotal >= purchaseAmount