Linkable dashboard views

The dashboard now keeps the open project and tree in the URL, so a single view — the Second Renaissance current reality tree, say — can be linked to directly instead of "open the dashboard, pick the project, click Reality", and Back and Forward walk the views visited. Stale links degrade rather than break: a tree the project has not modelled opens its overview, and an unknown project lands on the picker. The shape of the links is in docs/dashboard.md.