stream_data: Always initialize pin_to_top.

This simplifies the StreamSubscription type, and parallels how we
always initialize every other property that isn’t in
NeverSubscribedStream (audible_notifications, desktop_notifications,
email_notifications, push_notifications, wildcard_mentions_notify,
color, is_muted).  email_address was already optional.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

web/src/stream_data.ts

@@ -736,6 +736,7 @@ export function create_sub_from_server_data(
         render_subscribers: !realm.realm_is_zephyr_mirror_realm || attrs.invite_only,
         newly_subscribed: false,
         is_muted: false,
+        pin_to_top: false,
         desktop_notifications: null,
         audible_notifications: null,
         push_notifications: null,