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
