RFC 74 - Worker, workerType, and provisioner explorer UI

Proposal

As discussed today in SF..

We need a tool for examining and manipulating running workers. It is based on the model and data gathered in #71. It would have the following pages:

a list of provisioners, linking to..
for a provisioner, a list of workerTypes, each with its worker count and pending count
an information page for a single worker, containing:
- worker.extra and other metadata
- action buttons (see #71)
- recently claimed tasks and their statuses

Note that there is no page listing all workers of a worker type, and note that there is no single-workertype page. We could add those later.