Add AJAX /cart_link

This is part 1 of 2, which the other one is in spree_auth_devise.

With this change, we can cache the entire page and have the only dynamic content on the page (the link to the cart) loaded through AJAX.
Whole-page cache OrdersController#edit

This makes loading /cart on subsequent requests much faster
No longer cache Spree::ProductsController#show based on current user

The only reason this was done because the login information needed to be dynamic. With recent changes in spree_auth_devise, this information is retrieved using an AJAX request (like the link_to_cart info). This means then that we can cache the entire page with no worries.