refs https://github.com/TryGhost/Team/issues/1524
- This enables admins in the ghost admin to have an overview of the total posts/members associated with a newsletter.
- Follows the `?include=count.x` convention used by other resources
refs https://github.com/TryGhost/Team/issues/1533
- Retrieves one newsletter
- Makes the newsletter resource consistent with the other resources
- Solves an issue with the admin expecting the route to exist
refs https://github.com/TryGhost/Team/issues/1463
- This enables listing, creating and editing newsletters
- The tests are commented out as the permissions will be added in a follow-up commit