Laminar NixOS module

[Chris Warburton]

Laminar looks like a much simpler and less flaky system than Hydra. We
should package it as a NixOS module.

Some thoughts:

 - There are a few options, read from environment variables. This is
   easy to set in a systemd service. Hence we don't need a config file.
 - Most of the "real" configuration is via scripts in the cfg subdir of
   Laminar's "home" (/var/run/laminar?). I don't think it's rebuilding
   the service each time these are changed; hence they should live
   somewhere outside of Nix's control. I propose that the options should
   include a path to use for this cfg directory, and that we symlink
   that path into the Laminar home dir. That way the config can be kept
   in version control, or "deployed" from git, or just live in a home
   dir.