More efficient listing of roles

Reduces number of loops from 2 to 1.

app/views/admin/registrations/index.html.haml

@@ -28,10 +28,9 @@
               = registration.id
             %td
               = registration.name
-              - @conference.roles.each do |role|
+              - registration.user.roles.each do |role|
-                - role.users.each do |user|
+                - if role.resource == @conference
-                  - if user == registration.user
+                  %span.label.label-info= role.name.titleize
-                    %span.label.label-info= role.name.titleize
             %td
               = registration.email
             %td