Django authentication backends based on the use of Keycloak and OpenID
Connect have been moved to the swh-auth module and turned into generic
backends in order to easily reuse them in any Django application.
So remove all code and tests that have been moved in swh-auth and
configure swh-web to use the generic authentication backends.
That diff will fail to build until a new swh-auth version is tagged.
Related to T3150
Depends on D5366