Backend playlist token

[kernicPanel]

Purpose

Implementation for #2286

Fixes #1342

[AntoLC]

I don't think my review has too much weight.

[lunika]

Does it work if you replace with this ?

playlist_exists = models.Playlist.objects.filter(id=playlist_id).exists()
            try:
                return (
                    playlist_exists
                    and view.get_queryset()
                    .filter(id=view.get_object_pk(), playlist__id=playlist_id)
                    .exists()
                )
            except FieldError:
                return playlist_exists and (
                    view.get_queryset()
                    .filter(id=view.get_object_pk(), video__playlist__id=playlist_id)
                    .exists()
                )

[lunika]

Here you are using pk= and line 173 you are using id=. Both are good IMO, we should probably choose the one we use everywhere.