Clean up common user modules

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

users/common/options.nix

@@ -0,0 +1,31 @@
+{ lib, ... }:
+
+let
+  userOptions = with lib; with types; { config, ... }: {
+    options.email = mkOption {
+      type = nullOr str;
+      description = "Email address of the user.";
+    };
+
+    options.fullName = mkOption {
+      type = nullOr str;
+      description = "Full name of the user.";
+    };
+
+    options.wallpaper = mkOption {
+      type = path;
+      description = "Path to the user's wallpaper.";
+    };
+
+    options.base16Scheme = mkOption {
+      type = path;
+      description = "Base16 scheme to use for the user's color palette.";
+    };
+  };
+in {
+  options = with lib; with types; {
+    users.users = mkOption {
+      type = attrsOf (submodule userOptions);
+    };
+  };
+}