misc improvements to modularity

modules/features/shells.nix

@@ -11,6 +11,6 @@
     programs.${config.features.shells.defaultShell}.enable = true;
 
     users.defaultUserShell = pkgs.${config.features.shells.defaultShell};
-    users.users."grey".shell = pkgs.${config.features.shells.defaultShell};
+    users.users."${config.info.userName}".shell = pkgs.${config.features.shells.defaultShell};
   };
 }