Show conference changelog

Use load_and_authorize_resource in versions controlller
Add conference specifc route to revision history page
Users with role can view revision_history only for the versions they have access to
Handle versions where conference_id is not set (records before papertrail was introduced)

config/routes.rb

@@ -100,6 +100,8 @@ Osem::Application.routes.draw do
           patch :update_conference
         end
       end
+
+      get '/revision_history' => 'versions#index'
     end
 
     get '/revision_history' => 'versions#index'