content [nfc]: Add TODO for one place we don't yet have "(guest)" markers

A quick test showed that the web app does this, so we should to. It
would be kind of annoying to get it to work here, so it might be
right to just not bother in this legacy codebase, and be sure to do
it in zulip-flutter.

Author: chrisbobbe

src/webview/js/rewriteHtml.js

@@ -145,6 +145,7 @@ const rewriteHtml = (auth: Auth, element: Element | Document = document) => {
   rewriteImageUrls(auth, element);
   rewriteTime(element);
   rewriteSpoilers(element);
+  // TODO add "(guest)" marker on @-mentions
 };
 
 export default rewriteHtml;