Setup basic Revision History page

Sets up paper_trail tracking in important models

and add conference_id to versions for storing conference_id
of conference related objects  as metadata.This will help in querying
for versions related to conferences.

Fix issues with factories & event types initialization.Import paper_trail testing helpers
Fix bug: Creating an event creates a useless version with event update

Add revert and view changes features to revision history

app/models/comment.rb

@@ -13,6 +13,8 @@ class Comment < ActiveRecord::Base
   # NOTE: Comments belong to a user
   belongs_to :user
 
+  has_paper_trail on: [:create, :destroy], meta: { conference_id: :conference_id }
+
   # Helper class method that allows you to build a comment
   # by passing a commentable object, a user_id, and comment text
   # example in readme
@@ -58,4 +60,8 @@ class Comment < ActiveRecord::Base
   def send_notification
     EventCommentMailJob.perform_later(self)
   end
+
+  def conference_id
+    commentable.program.conference_id
+  end
 end