Implement quantity field for home and shop page

[bobbingwide]

We need to continue to support the ability to choose the quantity when adding to the cart from the home or shop page.

Current implementation

https://github.com/bobbingwide/woocommerce-thumbnail-input-quantities
doesn't actually work. It displays the quantity field but doesn't pass the value through.

Proposed solution

https://wordpress.org/plugins/quantity-field-on-shop-page-for-woocommerce
works. But

• it produces Notices when WP_DEBUG true
• it has more code than we need

So, rather than change it, we can just copy the relevant code to the child theme.

Since in the current implementation the add to cart functionality takes the user to the Shopping cart
we don't need any special AJAX logic to update the values or messages on the current page.
So all we need to do is to implement the logic for the filter 'woocommerce_loop_add_to_cart_link.

The wordpress.org plugin called quantity-field-on-shop-page-for-woocommerce