Add the current category to the product page

While the product tagging lists all the categories a product is categorised under, it is also necessary to show the tagging for the current category that the user is in, when viewing a product.
This relies on the `id_category_current` variable being set. It can be viewed at https://github.com/PrestaShop/PrestaShop/blob/master/controllers/front/ProductController.php#L598