Add live dashboard for teachers in the context of an exercise

This commit also adds the fundamentals for ActionCable

app/models/comment.rb

@@ -1,8 +1,12 @@
 class Comment < ApplicationRecord
   # inherit the creation module: encapsulates that this is a polymorphic user, offers some aliases and makes sure that all necessary attributes are set.
   include Creation
+  include ActionCableHelper
+
   attr_accessor :username, :date, :updated, :editable
 
   belongs_to :file, class_name: 'CodeOcean::File'
   belongs_to :user, polymorphic: true
+
+  after_save :trigger_rfc_action_cable_from_comment
 end