Add folder structures to collections and the Admin UI

[janus-reith]

Similar to my request to add tree structures, it would be great to allow organizing documents in folders.

In contrast to the trees, folders would need additonal data, I wonder what the best way would be to schematize these.
Making them stub documents within the collection would align with the existing child->parent relation, but that's probaby not a good idea, especially since some collections define required fields.
It would probably make sense to store folders as a separate Entity

This might in some parts align with considerations for tree structures, I however made folders a separate discussion since the former could probably be implemented without requiring changes to the data model.

[mysteren]

#1961 (comment)