Add apache conf for jwchat, separate from fbx site conf.

This adds a jwchat.conf file under /etc/apache2/conf-available and enables it. It's a similar setup to owncloud. Now jwchat is available at http://freedombox.local/jwchat and https://freedombox.local/jwchat.

I didn't remove the old jwchat conf from the fbx.conf site yet, because plinth links to that URL. If this change looks good then I'll update the link in plinth, so the old stuff can be removed.