Prosody service

Here is my first approach to pack prosody as a service. Feel free to add some feedback :)
Hmmm... the split between nixpkgs and nixos is that nixpkgs just creates the package, and nixos configures it declaratively. Nixpkgs is used standalone as well, so hardcoding the configuration like this make it not usable by pure nixpkgs users that don't have easy root access, no?