Guest control: Cleaned methods involved in fetching/handling

- Moves guest user caching from User class to app container for
  simplicity.
- Updates test to use simpler $this->users->guest() method for
  consistency.
- Streamlined helpers to avoid function overlap for simplicity.
- Extracted user profile dropdown while doing changes.

app/Settings/SettingController.php

@@ -34,7 +34,7 @@ class SettingController extends Controller
         return view('settings.' . $category, [
             'category'  => $category,
             'version'   => $version,
-            'guestUser' => User::getDefault(),
+            'guestUser' => User::getGuest(),
         ]);
     }