Make NixOS config

[rien]

Because we want to be able to setup zauth in a simple and reliable way, it makes sense to make a flake which does a few things:

• Provide zauth as a package (in nixpkgs)
• Creates a sytemd service running zauth
• Creates an nginx vhost to serve zauth and its static files
• Creates a postgres database with a user for zauth
• Creates a postfix mail server which forwards <username>@zeus.ugent.be to the email linked to that username using postfix' psgql_table

The accentor flake can serve as an inspiration for this.

[chvp]

This can probably be closed, since the decision was made to not use nix?