refs https://github.com/TryGhost/Team/issues/953
- Emails posts should be not explorable by the rest of the frontend similarly to the draft or scheduled posts. Email posts should also keep the content gating, so that specific parts of content can still be gated based on the post's visibility setup
- A separate frontend router was chosen to implement this part of the system instead of a moutable express app due to increased complexity to introduce the latter approach.
- All "sent" email-only posts will be accessible through the `/email/:slug/` route
