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

db/migrate/20160524050538_add_conference_id_to_versions.rb

@@ -0,0 +1,5 @@
+class AddConferenceIdToVersions < ActiveRecord::Migration
+  def change
+    add_column :versions, :conference_id, :integer
+  end
+end

db/migrate/20160614145614_add_id_to_users_roles.rb

@@ -0,0 +1,5 @@
+class AddIdToUsersRoles < ActiveRecord::Migration
+  def change
+    add_column :users_roles, :id, :primary_key
+  end
+end

db/schema.rb

@@ -483,7 +483,7 @@ ActiveRecord::Schema.define(version: 20160704092023) do
   add_index "users", ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true
   add_index "users", ["username"], name: "index_users_on_username", unique: true
 
-  create_table "users_roles", id: false, force: :cascade do |t|
+  create_table "users_roles", force: :cascade do |t|
     t.integer "role_id"
     t.integer "user_id"
   end
@@ -529,6 +529,7 @@ ActiveRecord::Schema.define(version: 20160704092023) do
     t.text     "object"
     t.text     "object_changes"
     t.datetime "created_at"
+    t.integer  "conference_id"
   end
 
   add_index "versions", ["item_type", "item_id"], name: "index_versions_on_item_type_and_item_id"