Standarize the order items API responses.

Problem/Motivation

There is not a consistent way to operate on a cart's items and get the modified cart back in response.

• Adding items to cart is only possible by posting to /cart/add, that can add to multiple orders, so it can not respond with an updated cart.
• Removing from cart responds with nothing.
• Updating order items responds with the whole cart.

This causes pain when trying to consume the API.

Proposed resolution

• Make the DELETE request to /cart/{commerce_order}/items/{commerce_order_item} endpoint respond with the updated cart.
• Create the ability to add items to a single order by making a POST request to /cart/{commerce_order}/items and make that endpoint respond with the updated cart.

API changes

• Response to DELETE requests to /cart/{commerce_order}/items/{commerce_order_item} will change. This should not be a braking change at most cases, since the current response is empty.
• The ability to make POST requests to /cart/{commerce_order}/items will be added.