unify usage of jQuery, re #993

resources/assets/javascripts/bootstrap/qr_code.js

@@ -7,8 +7,8 @@ $(document).on('click', 'a[data-qr-code]', function (event) {
 STUDIP.ready((event) => {
     $(document).on('click', '#qr_code .PrintAction', () => {
         // We must hide the other page elements for the print view functionality.
-        jQuery('#layout_wrapper').addClass('print-qrcode').hide();
-        jQuery('#qr_code').addClass('print-view');
+        $('#layout_wrapper').addClass('print-qrcode').hide();
+        $('#qr_code').addClass('print-view');
 
         // Now we can print:
         window.print();