[IMP] website_sale: allow optional products in shop

In shop, it opens the modal jus after having clicked on the 'Add to cart'. The modal contains the optional products.
This can be written in a single expression with `ctx = dict(context, active_id=product.product_tmpl_id.id)`.

And if you're updating a single key on an existing dict, don't use `.update`, use `[]` (it's just as clear and somewhat faster to boot)