compose: Add banner to go to conversation when replying in search view.

When a user is replying to a message while in a search view, we now warn
them that the full conversation is not visible and urge them to go to
the conversation they are composing to, so they can see the complete
conversation. On narrowing to that conversation, the banner is removed.

Fixes: #25893.

web/src/compose_reply.js

@@ -114,6 +114,7 @@ export function respond_to_message(opts) {
         topic,
         private_message_recipient: pm_recipient,
         trigger: opts.trigger,
+        is_reply: true,
     });
 }