Decrement available quantity as soon as an item is added to shopping cart

Should it instead double-check the inventory as part of the checkout process? On one site I'm involved in they regularly have people add items to their cart and leave them there for several days; this change could potentially stop would prevent others from completing their transaction.

While I do appreciate the idea of a customer walking around in a store with an item in their cart they haven't bought yet, in this analogy the customer wouldn't continue walking around the store for two weeks deciding whether they wanted to buy it.

Many sites that have tight inventory controls will have a timer that requires a visitor to complete the checkout within a certain amount of time, otherwise the item is made available for others. This is common with event ticket sales, e.g. concerts, etc.

I opened a separate issue for my idea from #11: ✨ Double-check inventory when checkout started Active

Configuration option to determine whether stock should be reserved upon order placement or when adding products to cart.

This is optional. If that is not the requirement for your site, do not configure it to reserve the item when it is added to the cart.

Rerolled the patch from #10 and added numerous fixes.

- The 'stock reservation upon adding to cart' feature is now fully optional
- Patch now works with the latest version
- Improved stock form functionality
- Added support for stock enforcement

My colleague (@arantxio.gottmers) and i have been working on this as we need it for our site and thought it would be beneficial to share it here as well.

I've noticed that it missed a import, so here is the adjusted patch.