refs https://github.com/TryGhost/Team/issues/949
- When post is marked as "email-only" we can send it out to the selected audience when publishing without making the post publicly available
- The feature is available for experimentation behind "email only" alpha flag available in labs
refs https://github.com/TryGhost/Team/issues/949
- Initializing PostsService with almost identical parameters is burdensome, having a single factory method in create instances is far more maintainable
refs https://github.com/TryGhost/Team/issues/949
- The post model handling related to newsletter sending and email recipient filter logic were duplicating across v3/v4(canary) APIs and it made sense to extract it into a posts service.
- This will allow for a central place to handle about to land logic for email_only newsletter handling.
refs https://github.com/TryGhost/Team/issues/949
- The code is exactly the same in six (!) places. It's beyond unmaintainable to add another line to any of these place, which will be needed for `email_only` handling.
- The newly created posts service is a temporary, slightly better solution that complies with codebase's best practice of extracting new services using class with DI pattern
