Support serializable toolbar

Hi @scuml,

The toolbar is undergoing a refactor to support serializing panel data so that it can be persisted. This has involved some backwards incompatible API changes that third-party panels will need to support.

The full set of changes is available at: https://github.com/django-commons/django-debug-toolbar/pull/2138

Please raise questions on that PR or open an issue on the repo.