Check for working .htaccess via AJAX

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

It wasn't working before: (master and this is the same)

![admin-settings](https://cloud.githubusercontent.com/assets/245432/5331090/4ca527a0-7e19-11e4-8c2d-2d634b38493b.png)