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 :-]

Hmmmm I made the last commit on top of master, so merging it into belenos branch may not be the right thing... if something needs to be fixed please tell me and I'll fix (e.g. redo the commit to belenos branch).