Checkout page should use query parameters to populate basket

[noisecapella]

The product id and optionally a coupon code should be provided by the page to populate the user's basket. The checkout page should read these parameters and PATCH /api/basket.

Acceptance Criteria:

• If the user loads the checkout page with a query parameter for the product id, the basket items should be emptied and the product for that id should be added to the basket.
• If a coupon code is in the query parameter it should be applied to the basket.
• If one of the query parameters is invalid and a 400 is received, the basket will be left as is.
• After a successful PATCH the query parameters should be removed from the URL