services: use consistent layout

Use the following order of definitions for all services:
- assertions
- configuration of other services
- environment.systemPackages
- tmpfiles
- own service
- users
- secrets

modules/joinmarket-ob-watcher.nix

@@ -54,6 +54,7 @@ in {
   };
 
   config = mkIf cfg.enable {
+    # Joinmarket is Tor-only
     services.tor = {
       enable = true;
       client.enable = true;