scaffold carts and create cart concern to retrieve/create cart via sess
establish one-to-many relation between Product-LineItem and Cart-LineItem. Carts will dependently destroy their line items, products will not be deleted if they are ref'd by a line-item via hook method in model
stuff gen'd by scaffold for line_items
created Add to Cart button, now adds line-items to current sess's cart, redirects to cart but doesn't show line-items... YET.