Added Demo mode

Demo mode allows to safely run a demo version of the app, with destructive features limited.

Some bugs were also fixed in this commit.

app/Services/DemoService.php

@@ -0,0 +1,11 @@
+<?php
+namespace App\Services;
+
+class DemoService {
+
+    public function isDemoEnabled(): bool {
+
+        return config('demo.is_enabled');
+
+    }
+}