Support of allow_named_servers setting

[szhem]

This PR adds support of allow_named_servers setting.

Previously, only default servers were supported for the users. Trying to start a named server for a given user led to the following behaviour:

• remove notebook for a named server passes a port it is running on to the jupyterhub
• jupyterhub assignes the port to the default notebook of a user and not to the named one
• so after a timeout jupyterhub still considers that there are issues with starting a remote notebook and returns error even if this remote notebook was started successfully

[szhem]

@jcrist Could you please see at the builds? They seem to fail because of jupyterhub versions

As far as I know jupyterlab.labhubapp is deprecated due to native integration.

What do you suggest to do in this case?