Add live dashboard for teachers in the context of an exercise

This commit also adds the fundamentals for ActionCable

app/assets/javascripts/application.js

@@ -14,6 +14,8 @@
 //= require turbolinks
 //= require pagedown_bootstrap
 //= require d3
+//= require rails-timeago
+//= require locales/jquery.timeago.de.js
 //
 // lib/assets
 //= require flash

app/assets/javascripts/base.js

@@ -23,3 +23,9 @@ $.fn.scrollTo = function(selector) {
     scrollTop: $(selector).offset().top - $(this).offset().top + $(this).scrollTop()
   }, ANIMATION_DURATION);
 };
+
+$.fn.replaceWithPush = function(a) {
+    const $a = $(a);
+    this.replaceWith($a);
+    return $a;
+};

app/assets/javascripts/cable.js

@@ -0,0 +1,13 @@
+// Action Cable provides the framework to deal with WebSockets in Rails.
+// You can generate new channels where WebSocket features live using the `rails generate channel` command.
+//
+//= require action_cable
+//= require_self
+//= require_tree ./channels
+
+(function() {
+  this.App || (this.App = {});
+
+  App.cable = ActionCable.createConsumer();
+
+}).call(this);

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"));
+                });
+            }
+        });
+    }
+});