Add live dashboard for teachers in the context of an exercise

This commit also adds the fundamentals for ActionCable

app/channels/la_exercises_channel.rb

@@ -0,0 +1,16 @@
+class LaExercisesChannel < ApplicationCable::Channel
+
+  def subscribed
+    stream_from specific_channel
+  end
+
+  def unsubscribed
+    stop_all_streams
+  end
+
+  private
+  def specific_channel
+    reject unless StudyGroupPolicy.new(current_user, StudyGroup.find_by(id: params[:study_group_id])).stream_la?
+    "la_exercises_#{params[:exercise_id]}_channel_study_group_#{params[:study_group_id]}"
+  end
+end