Issues/asdaiphone 4408

There are uses of the quantity editor where it makes sense to limit the minimum quantity to the step amount (e.g. 1 for units, 0.1 for kg) instead of 0. For instance, the product details view controller will show the editor for items not in the cart with an 'Add' button, but it does not make sense to add zero items to the cart, so we need to limit the minimum value to the step amount.

Most uses of the editor will be fine with a zero minimum so that is the default value.