This hardcodes the user, and has some other issues, so it is not yet
complete.

We follow mastodon's spec for automatic client registration, available
at both `/api/v1/apps` and a more reasonable `/oauth/client`. This
accepts a JSON POST with the client info and returns JSON with a
`client_id` and a `client_secret`, to be used with `/oauth/authorize`
and `/oauth/token`. It also, seemingly, requires returning an `id`
with unclear purpose.

The `/oauth/token` endpoint doesn't currently return a `me` field.