The kartei repository should contain all shared data (and nothing else), in particular (in no particular order):
- user names
- host names
- ssh public keys
- tinc public keys
- IP addresses

To use kartei in stockholm, it need to be augmented, i.e. stuff like `ci`, `cores`, etc. needs to be added (on a per-user basis?)

Instead of a separate repository, start by using `<stockholm/kartei>`.
- [x] move <stockholm/krebs/3modules/${username}> to kartei
- [x] move <stockholm/krebs/3module/external/${username}.nix> to kartei
- [x] 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)