Show flash data on basket page

Displays flash data on the full basket page in case a user gets sent
back with an error, such as when attempting to pay an order that does
not yet exist.

Also copies Bootstrap's alert-danger styles, since October calls it error.
Add order confirmation page

Adds a page to be displayed on successful payment with space for a
success message. Will double up as an order viewer too, taking the
order ID as a url parameter.
Fix checkout from mini basket

Change the checkout link to a button that calls the onCheckout method,
fixing an issue where the order wasn't being created before attempting
payment. Also updates the payment and product page parameters.