topic_list: Harden topic clicks against propagation.

I saw some profiling traces where it appears multiple copies of this
click handler got processed when clicking on a topic in the left
sidebar.

I suspect that was a profiling artifact, but the intent is for
on_topic_click to fully process clicks on topics in the sidebar, and
we should write the code to say that.

web/src/topic_list.ts

@@ -359,6 +359,7 @@ export function initialize({
             on_topic_click(stream_id, topic);
 
             e.preventDefault();
+            e.stopPropagation();
         },
     );