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)

lib/tasks/version.rake

@@ -1,7 +1,7 @@
 namespace :data do
   desc 'Sets conference_id in all pre-existing PaperTrail::Version objects'
   task set_conference_in_versions: :environment do
-
+    ids_with_failure = []
     PaperTrail::Version.where(conference_id: nil, item_type: %w[Conference Event]).each do |version|
       # All pre-existing versions are either of Conference or Event
       if version.item_type == 'Conference'
@@ -9,13 +9,25 @@ namespace :data do
 
       elsif version.item_type == 'Event'
         event = (version.item || version.reify || version.next.reify)
-        if event.try(:program)
-          version.update_attributes(conference_id: event.program.conference_id)
-        else
-          puts "Setting conference_id value failed for PaperTrail::Version object with ID=#{version.id}"
-        end
+
+        conference_id = if event.try(:program)
+                          event.program.conference_id
+                        # Event had attribute conference_id before it was replaced with program_id
+                        elsif version.changeset[:conference_id]
+                          version.changeset[:conference_id].second
+                        elsif version.changeset[:program_id]
+                          version.changeset[:program_id].second
+                        elsif version.object && (object = YAML.safe_load(version.object))
+                          object['conference_id']
+                        else
+                          ids_with_failure << version.id
+                          puts "Setting conference_id value failed for PaperTrail::Version object with ID=#{version.id}"
+                          nil
+                        end
+        version.update_attributes(conference_id: conference_id)
       end
     end
     puts 'All done!'
+    puts "IDs with failures: #{ids_with_failure}" if ids_with_failure.any?
   end
 end