Bug: Negotiation debug tool doesn't visualize the constraint tree

The negotiation debugger introduced by https://github.com/open-rmf/rmf_site/pull/336 was meant to visualize nodes in the constraint tree, allowing users to step through the negotiation process and visualize how the planner is functioning, similar to the [mapf-viz](github.com/open-rmf/mapf) tool. The PR for this feature was [never completed](https://github.com/open-rmf/rmf_site/pull/419).