Restructure folders, add mail host

The entire file structure has been reorganised into stable and unstable
to separate deployments running on either channel. A `mail` host running
Simple Nix Mailserver has also been added for testing to see if it's a
good alternative to soverin as well as SMTP for other services.

stable/config/hosts/mail/hardware-configuration.nix

@@ -0,0 +1,41 @@
+# Do not modify this file!  It was generated by ‘nixos-generate-config’
+# and may be overwritten by future invocations.  Please make changes
+# to /etc/nixos/configuration.nix instead.
+{ config, lib, pkgs, modulesPath, ... }:
+
+{
+  imports =
+    [ (modulesPath + "/profiles/qemu-guest.nix")
+      (modulesPath + "/profiles/minimal.nix")
+    ];
+
+  boot.initrd.availableKernelModules = [ "ata_piix" "virtio_pci" "virtio_scsi" "xhci_pci" "sd_mod" "sr_mod" ];
+  boot.initrd.kernelModules = [ ];
+  boot.kernelModules = [ ];
+  boot.extraModulePackages = [ ];
+
+  fileSystems."/" =
+    { device = "rpool/safe/root";
+      fsType = "zfs";
+    };
+
+  fileSystems."/home" =
+    { device = "rpool/safe/home";
+      fsType = "zfs";
+    };
+
+  fileSystems."/var" =
+    { device = "rpool/safe/var";
+      fsType = "zfs";
+    };
+
+  fileSystems."/nix" =
+    { device = "rpool/local/nix";
+      fsType = "zfs";
+    };
+
+  swapDevices =
+    [ { device = "/dev/disk/by-uuid/9c3c66f5-bf5a-4a2a-88a2-fc2ef312d7ef"; }
+    ];
+
+}