logo url no longer overrideable

[minrk]

The logo URL was previously controlled by default_url, but #168 has hardcoded the value to the nbclassic tree view root. As far as I can tell, it can no longer be overridden, as nbclassic_tree is always defined when running with jupyter_server.

I don't quite follow the discussion in #167 - the logo link is meant to go to the default URL for the server, not the tree root which is only the default default URL, but is meant to be able to be overridden. The tree root in the current view already has a link at the root:

This has been reported as a bug in jupyterhub at jupyterhub/jupyterhub#4220 where the expected behavior of setting default_url no longer has the desired effect.

[echarles]

cc/ @mcrutch