Make website output relocateable

With this change the website is prepared to be served from a
sub-directory on the webserver, e.g. from Gitlab Pages.

To archive this we need to do the following:

* Load our static assets from a relative URL.
* Make sure thumbnails and images are loaded from a relative URL.

But: If running from the live-server media is presented from a slightly
different URL. In this case we do not want to modify the thumbnail URLs.