Add grafana, A Graphite & InfluxDB Dashboard and Graph Editor

This is nodejs npm grunt based javascript frontend package, that has to be built with grunt. I created  separate `package.nix` file with all npm build dependencies. While this is suboptimal(we already have `pkgs/top-level/node-packages-generated.nix`), i think it's better this way. What do you think @shlevy @garbas ?