Remove message_id from index when marking it as read

Author: kaustubh-nair

zulipterminal/model.py

@@ -287,6 +287,7 @@ def toggle_message_star_status(self, message: Dict[str, Any]) -> None:
     def mark_message_ids_as_read(self, id_list: List[int]) -> None:
         if not id_list:
             return
+        self.index['unread_msgs'].difference_update(id_list)
         self.client.update_message_flags({
             'messages': id_list,
             'flag': 'read',