🔧(backend) configure Authorization Code authentication

Integrate 'mozilla-django-oidc' dependency, to support
Authorization Code flow, which is required by Agent Connect.

Thus, we provide a secure back channel OIDC flow, and return
to the client only a session cookie.

Done:
- Replace JWT authentication by Session based authentication in DRF
- Update Django settings to make OIDC configurations easily editable
- Add 'mozilla-django-oidc' routes to our router
- Implement a custom Django Authentication class to adapt
'mozilla-django-oidc' to our needs

'mozilla-django-oidc' routes added are:
- /authenticate
- /callback (the redirect_uri called back by the Idp)
- /logout

src/backend/core/tests/utils.py

@@ -1,15 +0,0 @@
-"""Utils for tests in the publish core application"""
-from rest_framework_simplejwt.tokens import AccessToken
-
-
-class OIDCToken(AccessToken):
-    """Set payload on token from user/contact/email"""
-
-    @classmethod
-    def for_user(cls, user):
-        """Returns an authorization token for the given user for testing."""
-        token = cls()
-        token["sub"] = str(user.sub)
-        token["email"] = user.email
-
-        return token