refs https://github.com/TryGhost/Team/issues/1552
- The API doesn't enforce a unique sort order but we infer the "default newsletter" based on ordering so we need to ensure a consistent and deterministic ordering behaviour
refs https://github.com/TryGhost/Team/issues/1469
Currently, all new members get auto subscribed to the default newsletter. This change adds same behavior with multiple newsletters by auto subscribing all available newsletters on site for new members(If flag is enabled).
Note: In future, this will also take into consideration the `subscribe_on_signup` flag for a newsletter to filter which newsletters should a member be auto-subscribed.
- adds newsletters service for working with newsletter data
- bumps `@tryghost/members-api` package which handles default subscription
- adds new test fixture/data for newsletters
