Use OpenID Connect to authenticate and authorize users in Django

As the decision has been made to use Keycloak as identity provider for Software Heritage, we need to add support for OpenID Connect into the Django authentication layer.

To do so, we could inspire from or directly use one of these existing Django applications:

• django-keycloak
• django-oidc, drf-oidc-auth, boss-oidc
• mozilla-django-oidc

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Migrated from T2048 (view on Phabricator)