Check for working .htaccess via AJAX

Fixes https://github.com/owncloud/core/issues/12650

@PVince81 @MorrisJobke Please review.

At some point I even wondered if it would be easier to try and access the source code from the browser, like "http://domain.tld/owncloud/lib/private/user.php". If that file is readable (no 404) it means htaccess badly configured.

That was just a thought, not a must.