Separate the (Python) server from the frontend in the source tree

[justvanrossum]

This is with @simoncozens: to clean up the Fontra source tree, and clearly separate the Python server source code from the front end code and assets, with the purpose of opening up more deployment options for Fontra, such as a serverless version.

Possible breakage that we must keep in mind

• Fontra Pak
• Fontra RCJK deployment

Steps:

• Rework URL scheme so the "project identifier" becomes a query key/value #1960
• ...next steps...

(To be edited...)