MDL-65518 badges: Admin settings warning

If the backpack is changed to Open Badges V2 backpack,
attempt the authenticate with the site settings and return a warning
if it fails.

badges/classes/output/external_backpacks_page.php

@@ -70,6 +70,7 @@ class external_backpacks_page implements \renderable {
             }
             $data->backpacks[] = $backpack;
         }
+        $data->warning = badges_verify_site_backpack();
 
         return $data;
     }