Calculating the URL is error-prone as it doesn't work when for example `public.php` is requested and might even be harmful as it could leak sensitive information.
Since `oc.js` is even available when maintenance mode is available it is better to use the global `OC.webroot` that is not controlled by any user.
* fix XMLHttpRequest - send after callback is registered
* check for correct type of value - it's a boolean
* return correct content-type in status.php
Using status.php for this.
I modified status.php to also show, whether we're in maintenance.
Checks every 20 seconds if maintenance is over, if yes: reload.
