Add syncthing base, git submodules

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

users/configs/syncthing/default.nix

@@ -0,0 +1,15 @@
+{ user ? throw "user argument is required" }: { ... }:
+
+{
+  networking.firewall = {
+    allowedTCPPorts = [ 22000 ];
+    allowedUDPPorts = [ 21027 22000 ];
+  };
+
+  home-manager.users.${user.name} = {
+    services.syncthing = {
+      enable = true;
+      extraOptions = [ "-no-default-folder" ];
+    };
+  };
+}