Add feed views relationships
Basically it adds the relationships from feed to content entities, using the node__feeds_item as pivote table to bridge feed entities to content(only) entities.
Also adds a groupwisemax relationship, which gets the first item belonging to specific feed, the first content item is determined by order that the user sets in relationship views plugin settings. To check it create a clone of feed view in /admin/content/feed
and add two relationships, first to get the (Feed)Feed representative feed item
, where order can be set, then the (Feed item)Content relationship
, so content fields can be fetched.
Closes #2953686