Show error message if not allowed to access admin page.

app/YtManagerApp/templates/YtManagerApp/settings_admin.html

@@ -4,8 +4,16 @@
 {% block body %}
 
     <div class="container">
+
         <h1>Admin settings</h1>
-        {% crispy form %}
+    
+        {% if not request.user.is_authenticated or not request.user.is_superuser %}
+            <div class="alert alert-danger" role="alert">
+                You must be an administrator to access this page!
+            </div>
+        {% else %}
+            {% crispy form %}
+        {% endif %}
     </div>
 
 {% endblock body %}