Implemented addition to and removal from cart. #6409

1. Addition to cart is checked in three places -:
    * Product Page
    * Related product on a product page
    * Wishlist
2. Removal from cart has been implemented for the Delete link which
appears for each product in the cart, as well as Cart hover menu.
3. Corrected the GA callback and timeout mechanism.
4. Shifted code to single place, i.e., base.jinja.