Update Controller to include update quantity in checkout

[lizkaraffa]

Allow users to change quantity of items in checkout

[mihaijoldis]

This should be the last step of checkout before payment. /store/checkout/payment/ Also if we add the quantity field update we should also add a way to remove items from teh cart just like on the /cart/ page.

When updates occur to the cart we should always redirect back to the initial /cart/ page just in case shipping needs to be recalculated