Run cron jobs via URL

Kanboard supports running cron jobs via CLI. There are hosting services
that don't offer CLI access, but they do offer calling a URL
periodically. This feature is often used as a CLI cron job replacement.

This commit adds a CronjobController called by "/cronjob" URL that will
execute cron jobs as they were executed via CLI. The URL has public
access, but is protected using the webhook token. The "/cronjob" URL
should be called via HTTPS.

app/ServiceProvider/AuthenticationProvider.php

@@ -143,6 +143,7 @@ class AuthenticationProvider implements ServiceProviderInterface
         $acl->add('FeedController', '*', Role::APP_PUBLIC);
         $acl->add('AvatarFileController', array('show', 'image'), Role::APP_PUBLIC);
         $acl->add('UserInviteController', array('signup', 'register'), Role::APP_PUBLIC);
+        $acl->add('CronjobController', array('run'), Role::APP_PUBLIC);
 
         $acl->add('ConfigController', '*', Role::APP_ADMIN);
         $acl->add('TagController', '*', Role::APP_ADMIN);