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/rudiger/services/redis.nix

@@ -1,10 +0,0 @@
-{ config, ...  }:
-{
-  services.redis = {
-    enable = true;
-    unixSocket = "/run/redis/redis.sock";
-    vmOverCommit = true;
-    unixSocketPerm = 770;
-    #requirePassfile = config.secrets.files.redis_pass.file;
-  }; 
-}