Issue #2842827: Offsite payment return fails on orders created from admin UI

[maciejzgadzaj]

Orders created from admin UI have $order->checkout_flow = NULL. This causes a fatal error when trying to add a new offsite payment for them in OffsitePaymentController::returnCheckoutPage() when trying to load the $checkout_flow_plugin. Same thing applies to OffsitePaymentController::cancelCheckoutPage().

See https://www.drupal.org/node/2842827