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
