If the order is allowed to retain a shipping_method_id, then subsequent
saves of the order will cause a new shipment to be initialised. Seems to
only happen for delivery shipping methods. This is undesirable because
fees for the new shipment will appear in the checkout summary, which is
not smart enough to recognise existing shipment fees and adjust the order
total accordingly.
Spree does not call after_<order.state> methods any more as of
https://github.com/spree/spree/pull/2557, so our #after_complete method
is never triggered and thus the order never reset.
This makes the condition:
```ruby
if current_order.andand.distributor == @order.distributor
```
in app/views/spree/orders/form/_update_buttons.html.haml return false
and as a result the "Back To Cart" button is not shown.
This commit resets the order (emptying the session[:order_id] and
creating a new order, aka. cart) right from the
CheckoutController#update rather than relying on infernal callbacks (of
what the Spree core team itself was unhappy about since long ago
https://github.com/spree/spree/issues/2488). There is the first place
where we know the order has been successfully completed.
