"/collections/{node_id}/tree endpoint structure improvement

[MRuecklCC]

Currently, the "/collections/{node_id}/tree endpoint returns a list of Nodes:

There are two issues with that:

• The parent_id fields of child nodes are null.

  They should either hold the uuid of their respective parent, or even better, we could just remvove the field, as it is redundant in the first place.

• Instead of returning a List[Node] we should simply return a singular Node.

  This makes it more clear, that all returned elements form a subtree of the queried toplevel node and unifies the datastructures.