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/target.rb

@@ -3,6 +3,8 @@ class Target < ActiveRecord::Base
 
   default_scope { order('due_date ASC') }
 
+  has_paper_trail ignore: [:updated_at], meta: { conference_id: :conference_id }
+
   def self.units
     {
       registrations: 'Registration',