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/subscription.rb

@@ -3,5 +3,7 @@ class Subscription < ActiveRecord::Base
   belongs_to :conference
   belongs_to :user
 
+  has_paper_trail on: [:create, :destroy], ignore: [:updated_at], meta: { conference_id: :conference_id }
+
   validates_uniqueness_of :user_id, scope: :conference_id, message: 'already subscribed!'
 end