Summary

Triggered by PLANET-7322 we should a fallback image when a featured image is not selected by the editor.

Requirements

• Add the fallback image on the repo (TBD)
• This should be applied in all post types
• When no featured image is selected use the default one on thumbnails (listing pages, articles and covers block)
• Use that fallback image also as an og:image in opengraph metadata
• To be refined: It may be easier to set it as a feature image and editors can replace it, instead of hardcoding the image in all use cases.