refs https://github.com/TryGhost/Team/issues/459
- fixed incorrect tracked variable name for the email preview address
- fixed incorrect access of `this.post` instead of `this.args.post`
- re-throw any error that is encountered so the cause is not silently lost
refs https://github.com/TryGhost/Team/issues/451
- disabled email tab when previewing a page (pages can't be emailed)
- added share preview url footer to browser tab
- added send preview footer to email tab
- added first pass at social tab contents
refs https://github.com/TryGhost/Team/issues/451
Start of new post preview prototype.
- adds preview button next to publish button
- opens new combined preview modal with desktop/email/social tabs
- desktop preview shows front-end preview url in iframe
- email preview fetches preview data as per the older email preview modal
- social preview to come later
