feat: add env variable for website base url

templates/_partials/_header.html.twig

@@ -1,5 +1,5 @@
 <div class="flex items-center justify-between">
-    <a href="https://www.ep-reisen.de" title="zur E&amp;P Website">
+    <a href="{{ website_base_url }}" title="zur E&amp;P Website">
         {% if booking_theme() == 'sbw' %}
             <img src="{{ asset('build/images/logo_sbw.svg') }}" alt="" class="h-10 my-2 w-auto">
         {% elseif booking_theme() == 'ser' %}