Fix active elements output not showing

Commit ad95bc9 changed poll.js
so that the final target URL is configured via an HTML data
attribute. However, the new data attribute was not added for active
elements, thus active elements could not load feedback to
the output element after the submission had been graded.

Fixes #1188

exercise/static/exercise/chapter.js

@@ -602,8 +602,10 @@
             // that is always included: <div class="quiz-submit-error alert alert-danger hide">
             const alerts = content.find('.alert-danger:not(.hide)');
             if (!alerts.length) {
-              var poll_url = content.find(".exercise-wait").attr("data-poll-url");
+              const poll_url = content.find(".exercise-wait").attr("data-poll-url");
               output.attr('data-poll-url', poll_url);
+              const ready_url = content.find(".exercise-wait").attr("data-ready-url");
+              output.attr('data-ready-url', ready_url);
 
               exercise.updateSubmission(content);
             } else if (alerts.contents().text()