Lighttpd package helper: fix default index html

Lighttpd currently comes directly from Ubuntu and the default index.html file refers to Ubuntu. This is the page you see if you install Lighttpd and browse to localhost:80. I changed the strings to say Trisquel GNU/Linux but it does refer to "upstream bug tracker" for bugs (i.e. ubuntu launchpad) because Trisquel doesn't list any lighttpd bugs.

By the way this is my first merge request (ever) so any comments/suggestions/advice/things to fix are very welcome :-]

I made a merge request for master branch but the helper is based on the belenos package (I just didn't know which branch to target, master or belenos) so hopefully you can merge to belenos if that's preferred. The index.html didn't change much for long time (I remember the Debian version) so the helper maybe applies to Trisquel 6 too.