Added inventoryPolicy check and min removed from Schema

[Akarshit]

Fix for #2743

1. Removed that min: 0 check from Product schema.
2. Added a condition to check for the inventoryPolicy before rejecting a order.

But this would cause the inventoryQuantity to be as negative as the no. of backorders on that variant. Is this the required behaviour?

[aaronjudd]

@Akarshit this looks good, but can you resubmit to marketplace (or we can release if there is a 1.4.3, but I'd rather just bring it into marketplace now as we're doing a fixes to the PDP). @spencern let's keep an eye and cherry-pick if needed to move this along.

[Akarshit]

@spencern Changed base and updated the PR.

[spencern]

👍 Works well, thanks for the PR.

[spencern]

This PR raises a few issues that are related but not caused by this PR. I'll create separate issues for these, but also list them here.

1. Toggling inventoryPolicy / "Allow Backorder" for a variant does not cascade that choice into it's options
2. Options do not have a switch that will permit toggling the inventory policy. I had to edit the inventoryPolicy for the options in the database directly in order to permit backordering on the default product.

[spencern]

#2911 is the new issue that I created to address the additional issues noticed here.