The kartei repository should contain all shared data, in particular (in no particular order):
- user names
- host names
- ssh public keys
- tinc public keys
- IP addresses

Instead of a separate repository, start by using `<stockholm/kartei>`.
- move <stockholm/krebs/3modules/${username}> to kartei
- move <stockholm/krebs/3module/external/${username}.nix> to kartei
- move <stockholm/krebs/3module/external.nix> to kartei
- update documentation
  - [https://github.com/krebs/retiolum/blob/master/README.md](https://github.com/krebs/retiolum/blob/master/README.md)