Dashboard cannot proxy Worker Dashboards in distributed cluster

**What happened**:

Set up a distributed Dask cluster (1 scheduler and 4 workers) each running on different hosts and accessed the dashboard:


Then tried to access the Worker Dashboards via the Info tab and received the following error:


**What you expected to happen**:

Expected to be able to access the worker dashboards successfully

**Anything else we need to know?**:

On initial attempt I got a different error advising that I needed to install the additional `jupyter-sever-proxy` package to enable worker dashboard proxying. However once installed you get the above error because the `jupyter-server-proxy` package enforces a whitelist that only has `localhost` and `127.0.0.1` in it by default.

There does not appear to be any way for users to customise this whitelist so it is hard to see how this feature is ever expected to work except for a fully local cluster i.e. all workers on same physical machine as the scheduler.

Ideally the Dask scheduler dashboard should automatically update the proxy whitelist whenever a new worker is registered.

**Environment**:

- Dask version: 2.17.0
- Python version: 3.8
- Operating System: OS X
- Install method: conda