[ENH] Toggle off the cluster manager section

The dask-labextension is the combination of two different concepts: cluster management (creation, scaling, and stopping) and dashboard viewing. I think that the dashboard section can be independently useful without the cluster manager. I propose adding a config flag to the [plugin.json](https://github.com/dask/dask-labextension/blob/master/schema/plugin.json) which would toggle on and off the cluster manager section.

For context, this came up at Saturn Cloud where we'd like to expose the dashboard components in jlab using dask-labextension, but we have our own mechanism for starting clusters that happens outside of jupyterlab.