Remove mixins, per-Authenticator LoginHandler classes

[minrk]

This PR simplifies oauthenticator quite a bit by reducing a lot of the boilerplate involved in defining/maintaining implementations.

• authorize_url, token_url, userdata_url are traits on base OAuthenticator (closes Refactor away some mixins #312)
• eliminates all env-only module-level config, moving everything to Authenticator traits (closes Entrypoints causes issues with JupyterHub. #286)
• removes never-released Authenticators that could be implemented via configuration of existing ones (AzureAd or Generic)

Implementation progress:

• github
• gitlab
• google
• bitbucket
• azuread
• azureadb2c (never released, removed and replaced with azuread config)
• auth0
• aws (never released, removed and replaced with generic config)
• cilogon
• generic
• globus
• mediawiki
• okpy
• openshift
• yandex (never released, removed and replaced with generic config)

In the future, I think we should perhaps promote more aspects of GenericOAuthenticator to the base class, and implement most of our providers via override of more granular methods than authenticate. I'm pretty sure several can be pretty basic already (openshift, okpy, auth0, bitbucket, google).

[consideRatio]

Wow great work going through this @minrk! I really would love to see this PR merged as I would greatly appreciate the reduced complexity that I've experienced both as a user and maintainer of this project. Thank you for reviewing this @NickolausDS!

Note that it is claimed that some of the removed authenticators was unreleased, I believe we did release some new authenticators as part of 0.10.0. Still, I think we should simply make a quick removal of them.

[NickolausDS]

I went through and tested the latest changes with both GitHub and Globus, and both worked great. It looks good to me!

[consideRatio]

@minrk this LGTM to the extent i can evaluate it, and i think it is an essential PR.

I'll go ahead and merge, thank you for review and testing @NickolausDS!