add Local variant

[minrk]

Proposed change

define a LocalFirstUseAuthenticator that creates system users. Most authenticators (OAuthenticator and friends) support this, and it generally consists of:

class LocalFirstUseAuthenticator(FirstUseAuthenticator, LocalAuthenticator):
    pass

Alternative options

reimplement local users another way

Who would use this feature?

users of FirstUseAuthenticator with the default Spawner or systemd or other system-user-requiring spawner. See jupyterhub/jupyterhub#2948

(Optional): Suggest a solution