Add live dashboard for teachers in the context of an exercise

This commit also adds the fundamentals for ActionCable

config/application.rb

@@ -28,5 +28,7 @@ module CodeOcean
     config.autoload_paths << Rails.root.join('lib')
     config.eager_load_paths << Rails.root.join('lib')
     config.assets.precompile += %w( markdown-buttons.png )
+
+    config.action_cable.mount_path = '/cable'
   end
 end