Update Bootstrap to v4.1, fix chosen.js and pagedown on multiple sites

app/views/exercises/feedback.html.slim

@@ -8,17 +8,17 @@ h1 = link_to(@exercise, exercise_path(@exercise))
   - if @feedbacks.nil? or @feedbacks.size == 0
     .no-feedback = t('user_exercise_feedback.no_feedback')
 
-  ul.list-unstyled.panel-group
+  ul.list-unstyled
     - @feedbacks.each do |feedback|
-      li.panel.panel-default
-        .panel-heading role="tab" id="heading"
+      li.card.mt-2
+        .card-header role="tab" id="heading"
           div.clearfix.feedback-header
             span.username = link_to(feedback.user.name, statistics_external_user_exercise_path(id: @exercise.id, external_user_id: feedback.user.id))
             - if feedback.anomaly_notification
               i class="fa fa-envelope-o" data-placement="top" data-toggle="tooltip" data-container="body" title=feedback.anomaly_notification.reason
             span.date = feedback.created_at
-        .panel-collapse role="tabpanel"
-          .panel-body.feedback
+        .card-collapse role="tabpanel"
+          .card-body.feedback
             .text = feedback.feedback_text
             .difficulty = "#{t('user_exercise_feedback.difficulty')} #{feedback.difficulty}" if feedback.difficulty
             .worktime = "#{t('user_exercise_feedback.working_time')} #{feedback.user_estimated_worktime}" if feedback.user_estimated_worktime