notification: Show wrong narrow notification for non locally echoed message.

Show "sent to different narrow" notification and other such notification by
notifications.notify_local_mixes for non locally echoed message sent by
current client.

With significant new comments added by tabbott.

Fixes: #11488.

frontend_tests/node_tests/server_events.js

@@ -9,6 +9,7 @@ global.stub_out_jquery();
 zrequire('message_store');
 zrequire('server_events_dispatch');
 zrequire('server_events');
+zrequire('sent_messages');
 
 set_global('blueslip', global.make_zblueslip());
 set_global('channel', {});