Hide iframes without using comments

If iframes previously were inside a comment, they ended the comment,
accidentally uncommenting code.

viewer/archive/archiveviewer.js

@@ -34,7 +34,7 @@ function replace_urls(text, url_before, url_after) {
 }
 
 function hide_iframes(text) {
-    return text.replaceAll("<iframe", "<!-- ").replaceAll("/iframe>", " -->")
+    return text.replaceAll("<iframe", "<div hidden ").replaceAll("/iframe>", "/div>")
 }
 
 function date_to_iso(value) {