MDL-69151 message_airnotifier: start output before accesskey request.

This allows debugging generated by the request to be shown on the
page.

message/output/airnotifier/requestaccesskey.php

@@ -61,6 +61,8 @@ if (strpos($CFG->airnotifierurl, AIRNOTIFIER_PUBLICURL) !== false ) {
     }
 }
 
+echo $OUTPUT->header();
+
 $manager = new message_airnotifier_manager();
 
 if ($key = $manager->request_accesskey()) {
@@ -72,6 +74,5 @@ if ($key = $manager->request_accesskey()) {
 
 $msg .= $OUTPUT->continue_button($returl);
 
-echo $OUTPUT->header();
 echo $OUTPUT->box($msg, 'generalbox ');
 echo $OUTPUT->footer();