refs https://github.com/TryGhost/Ghost/issues/10921
refs https://github.com/TryGhost/Ghost/pull/11068
- When there is no twitter or og image specified for a post or a page the new default falls back to appropriate site-wide twitter/og image or site's cover image.
- New rules of follback follow this logic:
post/page/amp = post.twitter_image || post.feature_image || settings.twitter_image || settings.cover_image;
post/page/amp = post.og_image || post.feature_image || settings.og_image || settings.cover_image;
