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/style.css

@@ -1,6 +1,12 @@
 body {
     color: #2898BD;
 }
+
+.hidden-before-load {
+    display: none !important;
+    visibility: hidden !important;
+}
+
 .input-field {
     border-radius: 10px;
     padding: 10px;