FLEX-613 add_portal_cart_listings Contact model method

add listings to a portal cart
It is but I didn't know they were "auto-created".  On dev, some contacts have portal enabled and these carts don't exist.  It was difficult for me to understand how they are supposed to work.  Also, there is functionality coming for creating a portal cart that is not auto-created.  I planned to have an 'else' for the 'if' to create the cart when it doesn't exist.