stream create: Overhaul create-stream add-subscribers UI.

The most notable change here is that when you are adding
subscribers to a stream as part of creating the stream,
you can now use the same essential pill-based UI for
adding users as we do when you edit subscribers for an
existing stream.

We don't try to exactly mimic the edit-stream UI or
implementation, since when you are adding subscribers
during create-stream, we are just updating a list in
memory, whereas in the edit-stream UI, we immediately
send info to the server.

Fixes #20499

static/js/zulip_test.js

@@ -14,3 +14,4 @@ export {last_visible as last_visible_row, id as row_id} from "./rows";
 export {cancel as cancel_compose} from "./compose_actions";
 export {page_params, page_params_parse_time} from "./page_params";
 export {initiate as initiate_reload} from "./reload";
+export {add_user_id_to_new_stream} from "./stream_create_subscribers";