labextension in proxy mode doesn't appear to work behind Open Ondemand - check_origin for websockets missing in proxy?

**What happened**:
We are attempting to run dask-jobqueue through Open OnDemand (https://openondemand.org/). This is working, _except_ for the labextension graphs etc. Once the cluster is launched, and the extension activated by inserting '/proxy/8787/status' in the extension search bar, clicking on the buttons produces blank tabs, and errors like this:
[W 2021-02-15 16:25:58.768 ServerApp] 403 GET /node/XXXXX/27554/proxy/8787/individual-task-stream/ws (XXX.XXX.XXX.XXX) 2.03ms referer=None

**What you expected to happen**:
I expect to get graphs etc instead of blank tabs.

**Minimal Complete Verifiable Example**:

```python
from dask.distributed import Client,Scheduler
from dask_jobqueue import SLURMCluster

cluster = SLURMCluster(cores=2,memory="4GB")
client = Client(cluster)
cluster.scale(2)
```
(then activate the extension and click a button)

**Anything else we need to know?**:
If I start the jupyterlab instance via ssh with port forwarding rather than OnDemand, it works, so it is something to do with running behind the proxy layers.
OnDemand does create a config.py as follows:
```python
c.NotebookApp.ip = '*'
c.NotebookApp.port = 8267
c.NotebookApp.port_retries = 0
c.NotebookApp.password = XXXXXXXX
c.NotebookApp.base_url = '/node/XXXX/8267/'
c.NotebookApp.open_browser = False
c.NotebookApp.allow_origin = '*'
c.NotebookApp.notebook_dir = '/home/ZZZZZ'
c.NotebookApp.disable_check_xsrf = True
```

Note particularly the allow_origin setting.

The problem appears to be that jupyter_server_proxy doesn't override the WebSocketHandler check_origin method in websocket.py/WebSocketHandlerMixin

If I add a check_origin method to this class which returns True, then everything appears to work.

My guess is that there should be a check_origin method in here very similar to the one in
jupyter_server/base/handlers.py/JupyterHandler but I don't know anything about the architecture of jupyter etc, so I don't know if that is actually the right place.

**Environment**:

- Dask version: 2021.2.0
- Python version: 3.7.4
- Operating System: Centos 8
- Install method (conda, pip, source): pip