Twig config improvements (#5923)

* deprecate direct kimai_config access, which was only used internally
* prevent access to system configs
* prevent access to most configs in sandbox environments

templates/activity/details.html.twig

@@ -93,7 +93,7 @@
     {% endif %}
 
     {% if stats is not null %}
-        {% set currency = kimai_config.customerDefaultCurrency %}
+        {% set currency = config('customerDefaultCurrency') %}
         {% if activity.project is not null %}
             {% set currency = activity.project.customer.currency %}
         {% endif %}