Add theme suggestions for the checkout order summary and completion message templates

[mglaman]

…er summary and completion message templates

[mglaman]

@bojanz the checkout message currently only receives the order. Should we also pass it the checkout_flow entity as well, then, so we can provide suggestion?

[mglaman]

I'm stupid.

  /** @var \Drupal\commerce_checkout\Entity\CheckoutFlowInterface $checkout_flow */
  $checkout_flow = $variables['order_entity']->get('checkout_flow')->entity;

Updated.