hotkey: Fix incorrect preventDefault for recent_topics hotkeys.

This logic correctly prevents the hotkeys implemented below it from
being active when "Recent topics" is open.  We expect to change some
parts of that soon (see #17685), but in any case, we should always
return false in the hotkey code when we don't process a key, so that
default browser behavior works.

This fixes browser shortcuts like Ctrl+C, Ctrl+V, etc. when recent
topics is loaded.

This bug was introduced in 1eafb1d8b3.

Thanks to ganpa3 for noticing this bug.

static/js/hotkey.js

@@ -756,7 +756,7 @@ export function process_hotkey(e, hotkey) {
     // open. These involve compose box hotkeys and hotkeys that can only
     // be done performed on a message.
     if (recent_topics.is_visible()) {
-        return true;
+        return false;
     }
 
     // Compose box hotkeys