Resolve "Make Helmholtz logo link to helmholtz.de"

Closes #23 (closed)

_includes/footer.html

@@ -9,7 +9,7 @@
 
     <div id="footer-content-container">
 
-        <a href="{{ base_url }}" class="logo-link">
+        <a href="https://www.helmholtz.de/" class="logo-link">
             <img    class="logo"
                     src="{{ '/assets/img/Helmholtz_Logo.svg' | relative_url }}"
                     alt="Helmholtz Logo">