Add live dashboard for teachers in the context of an exercise

This commit also adds the fundamentals for ActionCable

app/assets/javascripts/channels/la_exercises.js

@@ -0,0 +1,33 @@
+$(document).on('turbolinks:load', function() {
+    if ($.isController('exercises') && $('.teacher_dashboard').isPresent()) {
+
+        const exercise_id = $('.teacher_dashboard').data().exerciseId;
+        const study_group_id = $('.teacher_dashboard').data().studyGroupId;
+
+        const specific_channel = { channel: "LaExercisesChannel", exercise_id: exercise_id, study_group_id: study_group_id };
+
+
+        App.la_exercise = App.cable.subscriptions.create(specific_channel, {
+            connected: function () {
+                // Called when the subscription is ready for use on the server
+            },
+
+            disconnected: function () {
+                // Called when the subscription has been terminated by the server
+            },
+
+            received: function (data) {
+                // Called when there's incoming data on the websocket for this channel
+                let $row = $('tr[data-id="' + data.id + '"]');
+                if ($row.length === 0) {
+                    $row = $($('#posted_rfcs')[0].insertRow(0));
+                }
+                $row = $row.replaceWithPush(data.html);
+                $row.find('time').timeago();
+                $row.click(function () {
+                    Turbolinks.visit($(this).data("href"));
+                });
+            }
+        });
+    }
+});