Access to wp-cron.php should be disabled when Cavalcade is in use

Although it shouldn't happen, and should potentially be blocked at the server level, when Cavalcade is in use requests to `wp-cron.php` will still process any cron tasks.

As seen in #103, this currently doesn't actually work as it triggers a fatal error, but that just clouds error logs.

Ideally, Cavalcade should probably block/terminate-early the HTTP request.