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/users_role.rb

@@ -0,0 +1,12 @@
+class UsersRole < ActiveRecord::Base
+  belongs_to :role
+  belongs_to :user
+
+  has_paper_trail on: [:create, :destroy], meta: { conference_id: :conference_id }
+
+  private
+
+  def conference_id
+    role.resource_id
+  end
+end