Stock levels for BOM items being built-to-order

BOM items can be made and stocked, but if they are basically "kits" being "built to order" then the stock is generally zero ... while the users know they can make X based on the underlying parts. We can't pre-calculate this, and calculating it for every item on the page would cause a slow page load, but we could have a "Check for stock" button that calculates this on request (using the same on-request calculation that happens in exchequer for an individual BOM item. Holding the BOM data in CRM data tables would also be useful for other uses (like reporting) from time to time.