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/views/layouts/_admin_sidebar_index.html.haml

@@ -27,3 +27,8 @@
       = link_to(admin_users_path) do
         %span.fa.fa-user
         Users
+  - if can? :index, PaperTrail::Version.new(item_type: 'User')
+    %li
+      = link_to(admin_revision_history_path) do
+        %span.fa.fa-history
+        Revision History