ts: Migrate `stream_create_subscribers_data.js` to TypeScript.

While `user_id` from `page_params` is marked as `number | undefined`,
when fuctions in this file are called, `page_params.user_id` will
never be undefined. Verifying from all of callers of
`initialize_with_current_user`, this is because when user is able
to create and initialize a stream, this user is authenticated and
thus a `user_id` is always available. Therefore, adding an assertion
when undefined user id is detected to handle a confirmed error.

web/src/stream_create_subscribers_data.ts

@@ -1,21 +1,25 @@
+import assert from "minimalistic-assert";
+
 import {page_params} from "./page_params";
 import * as people from "./people";
+import type {User} from "./people";
 
-let user_id_set;
+let user_id_set: Set<number>;
 
-export function initialize_with_current_user() {
+export function initialize_with_current_user(): void {
     const current_user_id = page_params.user_id;
-    user_id_set = new Set();
+    user_id_set = new Set<number>();
+    assert(current_user_id !== undefined, "Current user's id is undefined");
     user_id_set.add(current_user_id);
 }
 
-export function sorted_user_ids() {
+export function sorted_user_ids(): number[] {
     const users = people.get_users_from_ids([...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() {
+export function get_all_user_ids(): number[] {
     const potential_subscribers = people.get_realm_users();
     const user_ids = potential_subscribers.map((user) => user.user_id);
     // sort for determinism
@@ -23,21 +27,21 @@ export function get_all_user_ids() {
     return user_ids;
 }
 
-export function get_principals() {
+export function get_principals(): number[] {
     // Return list of user ids which were selected by user.
     return [...user_id_set];
 }
 
-export function get_potential_subscribers() {
+export function get_potential_subscribers(): User[] {
     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) {
+export function must_be_subscribed(user_id: number): boolean {
     return !page_params.is_admin && user_id === page_params.user_id;
 }
 
-export function add_user_ids(user_ids) {
+export function add_user_ids(user_ids: number[]): void {
     for (const user_id of user_ids) {
         if (!user_id_set.has(user_id)) {
             const user = people.maybe_get_user_by_id(user_id);
@@ -48,7 +52,7 @@ export function add_user_ids(user_ids) {
     }
 }
 
-export function remove_user_ids(user_ids) {
+export function remove_user_ids(user_ids: number[]): void {
     for (const user_id of user_ids) {
         user_id_set.delete(user_id);
     }