closes https://github.com/TryGhost/Team/issues/1125
refs 3c822e0457
- Email-only is not considered a general availability feature and can be used without special flags.
- It allows to publish a new post type "email only" that only goes out as an email newletter and is available through an undescoverable URL (does not appear anywhere publicly similarly to preview posts) on the site.
refs https://github.com/TryGhost/Team/issues/990
- Relying on uuid instead of slug makes the posts less discoverable and partially soves discoverability through overriden robots.txt files
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
