fix: front-end improvements

makes some fields visible and other hidden depending on the result of
some API calls, e.g. visitor is logged in -> show "Log out" button

WebApp/wwwroot/css/site.css

@@ -2,6 +2,11 @@ html {
   font-size: 14px;
 }
 
+.hidden-before-load {
+    display: none !important;
+    visibility: hidden !important;
+}
+
 @media (min-width: 768px) {
   html {
     font-size: 16px;