html extensions

Added some explicit links to .html where appropriate. In both the public DESY test site and the corresponding review app this seems to work.

_includes/frontpage/title-content.html

@@ -4,7 +4,7 @@
 </div>
 <div class="services-box">
     <div class="services-title-buttons">
-        <a href="{{ '/cloud-platform' | relative_url }}" class="btn btn-light btn-lg text-dark">
+        <a href="{{ '/cloud-platform.html' | relative_url }}" class="btn btn-light btn-lg text-dark">
             <i class="fas fa-cloud"></i> Helmholtz Cloud
         </a>
         <a href="https://software.hifis.net" class="btn btn-light btn-lg text-dark">
@@ -13,7 +13,7 @@
         <a href="{{ '/you-and-hifis' | relative_url }}" class="btn btn-light btn-lg text-dark">
             <i class="fas fa-user-plus"></i> HIFIS for you
         </a>
-        <a href="{{ '/contact' | relative_url }}" class="btn btn-light btn-lg text-dark">
+        <a href="{{ '/contact.html' | relative_url }}" class="btn btn-light btn-lg text-dark">
             <i class="fas fa-question"></i> Support + Contact
         </a>
     </div>