Allow to map a Media specifying a "goto another Media" as action recycling the feature described in {T574}.
Must code:
[ ] create entity RelatedMedia
[ ] create query manager QueryRelatedMedia
[ ] backend: add Media picker in `media-edit.php`
[ ] API: expand `get-media.php` (and documentation? https://atlas.landscapefor.eu/api/)
[ ] frontend JS: show in Atlas Media popup
Bonus levels:
[ ] frontend SEO: show