jupyterhub-singleuser as a Jupyter Server 2.0 extension

mostly a copy (fork) of singleuser app
using public APIs instead of lots of patching.

opt-in via `JUPYTERHUB_SINGLEUSER_EXTENSION=1`

related changes:

- stop running a test single-user server in a thread. It's complicated and fragile.
  Instead, run it normally, and get the info we need from a custom handler registered via an extension
  via the `full_spawn` fixture

setup.py

@@ -55,6 +55,7 @@ def get_package_data():
         'alembic/*',
         'alembic/versions/*',
         'event-schemas/*/*.yaml',
+        'jupyterhub/singleuser/templates/*.html',
     ]
     return package_data