[simple] link preferences on /about and /stats

We want to link the preferences from every page
(except of course the preferences page).

searx/templates/simple/base.html

@@ -47,6 +47,10 @@
         </div>
     {% endif %}
 
+    {% block linkto_preferences %}
+    {% from 'simple/icons.html' import icon_big %}
+      <nav id="linkto_preferences"><a href="{{ url_for('preferences') }}">{{ icon_big('menu-outline') }}</a></nav>
+    {% endblock %}
     {% block header %}
     {% endblock %}
     {% block content %}