Jupyterlab async and SSO bugs inside DLAB

[mcdoker18]

We experience two problems when we deploy the Jupyterlab in DLAB:

• The httpx library fails when there is the HTTP_PROXY environment variable.
  
• The SSO integration fails, because DLAB adds a prefix to a Jupyterlab URL. https://github.com/legion-platform/legion/blob/develop/legion/jupyterlab-plugin/legion/jupyterlab/handlers/oauth2.py#L26