GitLab

Use a mixin for LocalAuthenticator variant

c.LocalAuthenticator would also work due to inheritance,
but this is more specific and clearer

This uses the same pattern as in various OAuthenticators,
for example https://github.com/jupyterhub/oauthenticator/blob/master/oauthenticator/github.py#L180

fixes redirect issue; adds token query param; adds sign with secret key

Fix issues with audience validation

Add a basic JSONWebToken Authenticator Implementation