refactor(users/glittershark): Rename to grfn

Rename my //users directory and all places that refer to glittershark to
grfn, including nix references and documentation.

This may require some extra attention inside of gerrit's database after
it lands to allow me to actually push things.

Change-Id: I4728b7ec2c60024392c1c1fa6e0d4a59b3e266fa
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2933
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: lukegb <lukegb@tvl.fyi>
Reviewed-by: glittershark <grfn@gws.fyi>

users/grfn/system/home/home.nix

@@ -0,0 +1,20 @@
+{ config, pkgs, ... }:
+
+{
+  imports = [
+    (throw "Pick a machine from ./machines")
+  ];
+
+  # Let Home Manager install and manage itself.
+  programs.home-manager.enable = true;
+
+  # This value determines the Home Manager release that your
+  # configuration is compatible with. This helps avoid breakage
+  # when a new Home Manager release introduces backwards
+  # incompatible changes.
+  #
+  # You can update Home Manager without changing this value. See
+  # the Home Manager release notes for a list of state version
+  # changes in each release.
+  home.stateVersion = "19.09";
+}