Lighttpd: generate config file and add gitweb sub-service

This pull request is about making lighttpd easier to configure. Below is the shortlog, each commit has a bit more details.

Move lighttpd.nix to lighttpd/default.nix
lighttpd: generate a default config file
lighttpd: add services.lighttpd.mod_userdir option
lighttpd: add services.lighttpd.mod_status option
lighttpd: add gitweb as a sub-service