Make restrict quantity on cart form feature of node checkout configurable (see #705178)

Context: #705178: User can change the quantity on the cart screen

This issue presented a use case where a custom node should be attached to ever item ordered, and therefore changing the quantity from the cart was a bad thing because a user would not get to create a new node as qty changed from 1 to 2 for example. Essentially, they would be needlessly paying for an extra copy of the same thing.

However, this isn't the only use case; we have users create nodes with some information attached and they can order as many copies of that customized item as they'd like, but with the fix for the other use case committed users can no longer change the qty from their cart. It should be made configurable.

Patch attached adds a checkbox in the node checkout settings to do just this.