Define the initial NixOS configuration of webforge in a flake

[btlogy]

Fixes #31

Can be:

• tested with:

  nix flake check
  

• build with:

  nix build .#nixosConfigurations.webforge.config.system.build.toplevel
  

• further tested with:

  nixos-rebuild dry-activate --flake .#webforge --target-host root@webforge.tahoe-lafs.org
  

• deployed with:

  nixos-rebuild dry-activate --flake .#webforge --target-host root@webforge.tahoe-lafs.org
  

Edits:

1. The above commands requires this entry in the known_hosts:

   webforge.tahoe-lafs.org,135.181.155.146,2a01:4f9:c011:b882::1 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIKPX87odbiiSbMyxb2N0curagHWUqSDPBG/P6bQcVzjJ
   

2. For an additional 99 LoC, we could have CI here too:

   • Define the initial NixOS configuration of webforge in a flake with CI support #33

   I'll leave it to the reviewer to choose

[btlogy]

Closed in favor of #33