default_channel: Fix multiple modals opening.

Earlier, the `Add Channel` button was being focused even after the
modal was open, causing multiple modals to open on pressing enter
repeatedly.
This commit fixes the issue by removing the focus from the `Add
Channel` button after the modal is open by focusing the dropdown on the
modal.

web/src/settings_streams.ts

@@ -222,6 +222,9 @@ function show_add_default_streams_modal(): void {
         loading_spinner: true,
         on_click: add_default_streams,
         post_render: default_stream_post_render,
+        on_shown() {
+            $("#select_default_stream_0_widget").trigger("focus");
+        },
     });
 }