Add live dashboard for teachers in the context of an exercise

This commit also adds the fundamentals for ActionCable

app/helpers/action_cable_helper.rb

@@ -0,0 +1,15 @@
+module ActionCableHelper
+  def trigger_rfc_action_cable
+    if submission.study_group_id.present?
+      ActionCable.server.broadcast(
+        "la_exercises_#{exercise_id}_channel_study_group_#{submission.study_group_id}",
+        id: id,
+        html: (ApplicationController.render(partial: 'request_for_comments/list_entry',
+                                            locals: {request_for_comment: self})))
+    end
+  end
+
+  def trigger_rfc_action_cable_from_comment
+    RequestForComment.find_by(submission: file.context).trigger_rfc_action_cable
+  end
+end