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)

app/views/layouts/_user_menu.html.haml

@@ -44,7 +44,7 @@
     = link_to(admin_users_path) do
       %span.fa.fa-user
       Users
-- if can? :index, PaperTrail::Version.new(item_type: 'User')
+- if can? :index, PaperTrail::Version
   %li
     = link_to(admin_revision_history_path) do
       %span.fa.fa-history