refs https://github.com/TryGhost/Team/issues/1598
- added `<GhPostBookmark>` card for displaying a bookmark card style representation of a post
- updated designsandbox route to include it for easier styling without needing to constantly go through the publish flow to see changes whilst styling
- updated publish flow complete step to render a bookmark card if a post/page was published
- added `{{post-author-names}}` helper so the author name concatenation logic can be re-used across the posts list and bookmark component
