Default use jupyter-server-proxy with dask-labextension

**What happened**:

When trying to connect to a dask scheduler with the dask `jupyter-labextension` the buttons for the individual statistics are greyed out. The root cause is that the schedule URL is being constructed as

https://myjupyterhuburl.xyz/user/dfe2f561-ba35-4d44-b100-800857b4b683/128.219.134.85:4024/

(if the bokeh dashboard is available at **128.219.134.85:4024**), that is, the hostname and port is simply appended to the base URL, which I found out by looking at the Javascript console.

Instead, it should be

https://myjupyterhuburl.xyz/user/dfe2f561-ba35-4d44-b100-800857b4b683/proxy/128.219.134.85:4024/

Note, the base URL is different from a simple jupyter notebook installation because it is launched from a jupyterhub server.

**What you expected to happen**:

Active (orange) buttons to see the plots.

**Workaround**:

Set the environment variable
```
DASK_DISTRIBUTED__DASHBOARD__LINK="proxy/{host}:{port}/status"
```

and the buttons become active. **Request: Can we make proxying the URL the default?**

**Minimal Complete Verifiable Example**:

N/A

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

The scheduler runs on an IP and port that is not localhost, and is not reachable from the browser location (only from the notebook server). Therefore, it needs to be proxied.

**Environment**:

- Dask version: `2.25.0+7.g174968f `
- Python version: 3.8.5
- Operating System: Linux x86
- Install method (conda, pip, source): conda