Spin off TreeController, distinct from DocumentSetController

[refs #64262110]

New routes are easy to understand:

GET    /documentsets/:id/trees/:treeId controllers.TreeController.show(id: Long, treeId: Long)
GET    /trees/:treeId/nodes            controllers.NodeController.index(treeId: Long)
GET    /trees/:treeId/nodes/:nodeId    controllers.NodeController.show(treeId: Long, nodeId: Long)
POST   /trees/:treeId/nodes/:nodeId    controllers.NodeController.update(treeId: Long, nodeId: Long)