Replace bare header redirect

This fixes a bug where the browser will fail to redirect away from
the login page if the client's browser doesn't allow for location
mutations with the 422 response code.

This issue only impacts SSO auth plugins.

scp/login.php

@@ -67,7 +67,7 @@ elseif ($_GET['do']) {
 elseif (!$thisstaff || !($thisstaff->getId() || $thisstaff->isValid())) {
     if (($user = StaffAuthenticationBackend::processSignOn($errors, false))
             && ($user instanceof StaffSession))
-       @header("Location: $dest");
+       Http::redirect($dest);
 }
 
 // Browsers shouldn't suggest saving that username/password