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/inbox_ui.ts

@@ -74,7 +74,7 @@ type StreamContext = {
     invite_only: boolean;
     is_web_public: boolean;
     stream_name: string;
-    pin_to_top: boolean | undefined;
+    pin_to_top: boolean;
     is_muted: boolean;
     stream_color: string;
     stream_header_color: string;