refs https://github.com/TryGhost/Team/issues/1433
- The `default` property stores whether a newsletter is set as default by the admin
- The `status` property stores whether a newsletter is archived or not
- The `recipient_filter` property is only storing whether a newsletter is "paid-only" or not for now, although it can be expanded to more specific filters in the future
- The `subscribe_on_signup` property stores whether a new member should be automatically signed up to the newsletter
- The `sort_order` property enables displaying the newsletter list in an order chosen by the admins
