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

jupyterhub/services/auth.py

@@ -243,7 +243,6 @@ class HubAuth(SingletonConfigurable):
             return 'http://127.0.0.1:8081' + url_path_join(self.hub_prefix, 'api')
 
     api_token = Unicode(
-        os.getenv('JUPYTERHUB_API_TOKEN', ''),
         help="""API key for accessing Hub API.
 
         Default: $JUPYTERHUB_API_TOKEN
@@ -253,6 +252,10 @@ class HubAuth(SingletonConfigurable):
         """,
     ).tag(config=True)
 
+    @default("api_token")
+    def _default_api_token(self):
+        return os.getenv('JUPYTERHUB_API_TOKEN', '')
+
     hub_prefix = Unicode(
         '/hub/',
         help="""The URL prefix for the Hub itself.