Showing satis index.html as index. Satisfy backend available at /admin/

First: The Code in "$app->get('/', function() use ($app) {...}" does not work, because the home.html.twig needs the 'repositories' variable.

Second:
In my case, we have a satis repository running and developers are using the index.html generated by satis. The current code does not allow access to that generated index.html.
So i moved the satisfy backend to /admin/ and added a small proxy for the index.html with a useable error message.

@ludofleury May this be something to merge in?