Autofocus the "App Code" input on the TOTP prompt during MFA gates after login

Summary: See downstream <https://phabricator.wikimedia.org/T229757>. The "autofocus" attribute mostly just works, so add it to this input.

Test Plan: As a user with TOTP enabled, established a new session. Saw browser automatically focus the "App Code" input on the TOTP prompt screen.

Differential Revision: https://secure.phabricator.com/D20703

src/applications/auth/factor/PhabricatorTOTPAuthFactor.php

@@ -194,6 +194,7 @@ final class PhabricatorTOTPAuthFactor extends PhabricatorAuthFactor {
       $control = id(new PHUIFormNumberControl())
         ->setName($name)
         ->setDisableAutocomplete(true)
+        ->setAutofocus(true)
         ->setValue($value)
         ->setError($error);
     }