Add dashboard for tahoe-lafs

This adds a detailed dashboard for tahoe-lafs running on one node.

It's a v1; I expect to amend it when we have real data.

It looks like this (the data in the screenshot is stale, so it does not have a lot going on):

Since JSON blobs are not super nice to review I'll just merge this and see.

Fixes privatestorageops#396.