Site error during "Show receipt" operation

This is because we shouldn't be able to print receipts for orders that are not yet placed.

Here's a test and fix for what happens when the jQuery.print library is not available.

lol forgot to commit the changes. Ignore #3.

I've got a major concern about this code thats in commerce_pos_receipt.

  Drupal.AjaxCommands.prototype.printReceipt = function (ajax, response, status) {
    $(response.content).print({
      globalStyles: false,
      stylesheet: drupalSettings.commercePosReceipt.cssUrl,
      deferred: $.Deferred().done(function(){
        // Sloppy, should probaby be nice and scalable, but that might never be needed
        $('.commerce-pos-receipt-button-done').click();
      })
    });
  };

The code in modules/receipt/js/commerce_pos_receipt.js overrides the default submit behaviour and stops the order being completed until the receipt is printed. This feels super wrong. We should only be printing the receipt once the order has been placed and that happens when you press the button click that this is deferring. This means that if something in browser goes wrong or the user never closes the receipt screen the order is never placed.

Tested locally and with simplytest.me. The patch works as expected where the 'Show Receipt' option doesn't display on orders in the 'Draft' state and displays for orders in the 'Completed' state.

Let's postpone this on #2935465: Receipt flow potential leaves order unplaced. That issue results in this one being fixed. Once that it done we should add the tests from this one.

Actually having this in before #2935465: Receipt flow potential leaves order unplaced would make it simpler to write good tests for that one. Back to rtbc.

I credited @shabana.navas for the review.