Cluster Model update

There is currently some churn going on in the interfaces for a generic distributed cluster info dict (cf. #78, #77 #73).

At the same time, this extension takes that information, and converts it to a `ClusterModel` dictionary, a JSON-serializable representation of the cluster information to display on the front end. This conversion happens here:
https://github.com/dask/dask-labextension/blob/b68342bcfb43f905d7b86588fb3083f917519b5a/dask_labextension/manager.py#L240-L290

And it produces a dictionary with the typescript interface here:
https://github.com/dask/dask-labextension/blob/b68342bcfb43f905d7b86588fb3083f917519b5a/src/clusters.tsx#L776-L820

Once the churn on the upstream side dies down, we should update the `IClusterModel` interface to be as close as possible to the generic cluster info dictionary, to which should help cut down on further confusion and maintenance.

n.b. This would be a breaking change, as both the client-side and the server-side would have to be updated with the new interface.