Add live dashboard for teachers in the context of an exercise

This commit also adds the fundamentals for ActionCable

db/migrate/20190213131802_add_indices_for_request_for_comments.rb

@@ -0,0 +1,6 @@
+class AddIndicesForRequestForComments < ActiveRecord::Migration[5.2]
+  def change
+    add_index :request_for_comments, :submission_id
+    add_index :request_for_comments, :exercise_id
+  end
+end