stream_popover: Fix type bug.

The variable `select_stream_id` was assumed to be of type number when
it actually was of type string. A strict equality check was performed later
down in the code between `select_stream_id` and `current_stream_id`
(which is of type number).

This commit converts the variable `select_stream_id` to type number.

web/src/stream_popover.js

@@ -414,6 +414,7 @@ export async function build_move_topic_to_stream_popover(
         send_notification_to_new_thread = send_notification_to_new_thread === "on";
         send_notification_to_old_thread = send_notification_to_old_thread === "on";
         current_stream_id = Number.parseInt(current_stream_id, 10);
+        select_stream_id = Number.parseInt(select_stream_id, 10);
 
         if (new_topic_name !== undefined) {
             // new_topic_name can be undefined when the new topic input is disabled when