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/stream_create_subscribers_data.js

@@ -0,0 +1,55 @@
+import {page_params} from "./page_params";
+import * as people from "./people";
+
+let user_id_set;
+
+export function initialize_with_current_user() {
+    const current_user_id = page_params.user_id;
+    user_id_set = new Set();
+    user_id_set.add(current_user_id);
+}
+
+export function sorted_user_ids() {
+    const users = people.get_users_from_ids(Array.from(user_id_set));
+    people.sort_but_pin_current_user_on_top(users);
+    return users.map((user) => user.user_id);
+}
+
+export function get_all_user_ids() {
+    const potential_subscribers = people.get_realm_users();
+    const user_ids = potential_subscribers.map((user) => user.user_id);
+    // sort for determinism
+    user_ids.sort((a, b) => a - b);
+    return user_ids;
+}
+
+export function get_principals() {
+    // Return list of user ids which were selected by user.
+    return Array.from(user_id_set);
+}
+
+export function get_potential_subscribers() {
+    const potential_subscribers = people.get_realm_users();
+    return potential_subscribers.filter((user) => !user_id_set.has(user.user_id));
+}
+
+export function must_be_subscribed(user_id) {
+    return !page_params.is_admin && user_id === page_params.user_id;
+}
+
+export function add_user_ids(user_ids) {
+    for (const user_id of user_ids) {
+        if (!user_id_set.has(user_id)) {
+            const user = people.get_by_user_id(user_id);
+            if (user) {
+                user_id_set.add(user_id);
+            }
+        }
+    }
+}
+
+export function remove_user_ids(user_ids) {
+    for (const user_id of user_ids) {
+        user_id_set.delete(user_id);
+    }
+}