Log mail delivery errors to database and add an UI for it

app/views/admin/users/_users.html.haml

@@ -17,7 +17,10 @@
         %td= link_to show_user(user), [:admin, user]
         - if FoodsoftConfig[:use_nick]
           %td= user.name
-        %td= user.email
+        %td
+          = user.email
+          - if user.mail_delivery_status.any?
+            = link_to t('.show_email_problems'), admin_mail_delivery_status_index_path(email: user.email), class: 'btn btn-warning btn-mini'
         %td= format_roles(user, true)
         %td= format_time(user.last_activity)
         %td= link_to t('ui.edit'), edit_admin_user_path(user), class: 'btn btn-mini'