Nextcloud OAuth 2.0 support

[LilianBoulard]

Proposed feature

For people with a self-hosted Nextcloud instance, it would be very convenient to be able to connect to JupyterHub using the already existing users.

The implementation should be quite straightforward, a first version is available in #321.

I believe the only significant difference from existing connectors is that, because Nextcloud is self-hosted, the instance URL should be passed by the user.

If it fits in the project's scope, please give me a green light and I'd be up for submitting a PR with an updated implementation.

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[minrk]

We aren't generally accepting new OAuthenticator subclasses in this repo. GenericOAuthenticator should work with just about any oauth provider via configuration. If you'd like to add the configuration required to use GenericOAuthenticator to the documentation, that would be very welcome!