favicon not explicitly specified for /tree (Notebook v7)

[mcrutch]

If Jupyter Server is configured at a base_url other than / and a different service is handling the / url, the favicon for Jupyter will not appear on the /tree in Notebook v7 because it looks for /favicon.ico
Scenario:

• c.ServerApp.base_url = '/Jupyter/'
• Notebook v7/Labv4/NBClassic
• nginx server running on 80/443 that forwards /Jupyter/ to jupyter server running in the background
• favicon.ico served by NGINX that is not Jupyter's favicon
• visit http:///Jupyter/tree

Outcome:

• Favicon on tab is what is served by NGINX, not what is served by Jupyter

Solution:

• Block similar to this on the /tree view that inserts the favicon explicitly via metadata just like JupyterLab does and Notebook v7 does for some sub-pages: 804cd3a