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.

config/hosts/wind/services/gitea.nix

@@ -1,16 +0,0 @@
-{ ... }:
-
-{
-  services.gitea = {
-    enable = true;
-    domain = "git.graven.dev";
-    rootUrl = "https://git.graven.dev";
-    enableUnixSocket = true;
-    cookieSecure = true;
-    appName = "Graven Gitea";
-    settings = { "ui" = { "DEFAULT_THEME" = "arc-green"; }; };
-    database = {
-      type = "postgres";
-    };
-  };
-}