Login and logout support for lighttpd-turris-auth

We want to use https://gitlab.nic.cz/turris/lighttpd-turris-auth as generic login page for multiple services running on the router. These include services such as reForis that are ours and thus can contain our specific logout button but it also contains services such as netdata that we have no control over and thus we can't inject logout button there (well we could but should we?). Webapps is our top level page and thus it makes sense that this page would contain logout button and login state visualization in general.