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/difficulty_level.rb

@@ -2,7 +2,10 @@ class DifficultyLevel < ActiveRecord::Base
   belongs_to :program
   has_many :events, dependent: :nullify
 
+  has_paper_trail ignore: [:updated_at], meta: { conference_id: :conference_id }
+
   validates :title, presence: true
+
   validates :color, format: /\A#[0-9A-F]{6}\z/
 
   before_validation :capitalize_color
@@ -12,4 +15,8 @@ class DifficultyLevel < ActiveRecord::Base
   def capitalize_color
     self.color = color.upcase if color.present?
   end
+
+  def conference_id
+    program.conference_id
+  end
 end