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/sponsor.rb

@@ -2,6 +2,8 @@ class Sponsor < ActiveRecord::Base
   belongs_to :sponsorship_level
   belongs_to :conference
 
+  has_paper_trail ignore: [:updated_at], meta: { conference_id: :conference_id }
+
   mount_uploader :picture, PictureUploader, mount_on: :logo_file_name
 
   validates_presence_of :name, :website_url, :sponsorship_level