Update to use OIDC

[nothingface0]

As investigated here, django-allauth does not need to have a dedicated cern provider anymore, as the OpenID Connect (OIDC) one is sufficient.

This PR adapts the login procedure to OIDC.

Extra steps to be taken after this is merged:

• Add extra environment variables (after registering your application on the Application portal):
  1. CERN_SSO_REGISTRATION_CLIENT_ID
  2. CERN_SSO_REGISTRATION_CLIENT_SECRET
• Create a new SocialApplication:
  1. Go to /admin.
  2. Delete the existing one (SOCIAL ACCOUNTS -> Social applications).
  3. Create a new one:
• Migrate existing SocialAccounts:
  1. python manage.py shell to enter Django's interactive shell.
  2. Run:

     from allauth.socialaccount.models import SocialAccount
     sa = SocialAccount.objects.filter(provider="cern")
     for s in sa:
     if 'username' in s.extra_data:
         s.uid = s.extra_data["username"]
         s.save()