pre-order capability

Allow products to be marked to override "stock checkout" restrictions.

Most products we carry are older products that are limited in supply. What we have is all we'll have. For these items, having "stock checkout" set to "no" works great. However, we have newer items that we can replenish, so bypassing "stock checkout" is needed.

It was suggested that we simply set the quantity to a high number as a workaround, but this has drawbacks: (1) Inaccurate inventory count; (2) still stop customers when product count drops to zero; (3) shows "in stock" when count > 0, but should be "pre-order" in cases such as taking orders for an unreleased product.