Stop querying Node.cachedDocumentIds
Stop writing node.cached_document_ids
Remove Node.cachedDocumentIds
Remove node.tree_id. Add tree.root_node_id.

This touches lots of files because of our foreign key checks. They
really force us to create interdependent components that ought to be
separate.

[finishes #74265114]
Nix wonky client-side hack: use Tree.rootNodeId
Be explicit for tree flexbox [finishes #74259950]