refs https://github.com/TryGhost/Team/issues/2506
`email` objects no longer contain a `html` field and the fallback logic in the email preview modal was failing resulting in a 404 from trying to fetch an email preview using the email id rather than a post id.
- added quick-fix to the preview modal logic to use `data.post_id || data.id` for generating the preview URL (previous logic never expected to reach the fallback when working with an email record)
3c695064bd