upgrade: MDL-19763 Improve the redirect that forces a DB upgrade when there are major changes

Also fix redirect, so 303 redirects don't need to fully initialise $OUTPUT.

lib/weblib.php

@@ -4941,6 +4941,8 @@ function redirect($url, $message='', $delay=-1) {
         //302 might not work for POST requests, 303 is ignored by obsolete clients.
         @header($_SERVER['SERVER_PROTOCOL'] . ' 303 See Other');
         @header('Location: '.$url);
+        echo bootstrap_renderer::plain_redirect_message($encodedurl);
+        exit;
     }
 
     // Include a redirect message, even with a HTTP redirect, because that is recommended practice.