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/_admin_sidebar.html.haml

@@ -143,8 +143,15 @@
       = link_to(admin_conference_roles_path(@conference.short_title)) do
         %span.fa.fa-group
         Roles
+
   - if can? :index, @conference.resources.new
     %li
       = link_to admin_conference_resources_path(@conference.short_title) do
         %span.fa.fa-pencil-square
         Resources
+
+  - if can?(:index, PaperTrail::Version.new(conference_id: @conference.id, item_type: 'Event')) || can?(:index, PaperTrail::Version.new(conference_id: @conference.id, item_type: 'Registration'))
+    %li{:class=> active_nav_li(admin_conference_revision_history_path(@conference.short_title))}
+      = link_to(admin_conference_revision_history_path(@conference.short_title)) do
+        %span.fa.fa-history
+        Revision History