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/cfp.rb

@@ -1,6 +1,7 @@
 # cannot delete program if there are events submitted
 
 class Cfp < ActiveRecord::Base
+  has_paper_trail ignore: [:updated_at], meta: { conference_id: :conference_id }
   belongs_to :program
 
   validates :program_id, presence: true
@@ -68,4 +69,8 @@ class Cfp < ActiveRecord::Base
     errors.
     add(:start_date, "can't be after the end date") if start_date && end_date && start_date > end_date
   end
+
+  def conference_id
+    program.conference_id
+  end
 end