refs https://github.com/TryGhost/Team/issues/459
- moved error text to the left of the input to avoid layout jump
- added aria attributes to mark field as invalid and associate error text
refs https://github.com/TryGhost/Team/issues/451
- Added mobile view to the post preview modal
- Replaced tabs text with icons
- Moved back button and added publish button
- Changed copy-link design to be aligned with link preview in portal
- Changed layout for social previews
- Added images to the Twittr and Facebook previews
- Added labels to the social previews
- Changed styling for the "Preview post" button in editor
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
