As the only consequence of isSystemUser is that if the uid is null then it's allocated below 500, if a user has uid = something below 500 then we don't require isSystemUser to be set. Motivation: https://github.com/NixOS/nixpkgs/issues/112647
Adding a new pleroma NixOS module and its associated documentation. co-authored-by: duponin <duponin@locahlo.st>