✨(backend) support silent login

Silent login attempts to re-authenticate the user without interaction,
provided they have an active session, improving UX by reducing manual auth.

It's an essential feature to really feel the SSO in La Suite.

A new query parameter, 'silent', allows the client to initiate a silent login.
In this flow, an extra parameter, 'prompt=none', is passed to the OIDC provider.

The requested flow is persisted in session data to adapt the authentication
callback behavior.

In a silent login flow, an authentication failure should not be considered as a
real failure. Instead, users should be redirected back to the originating view.
A silent login fails when user has no active session.

Why return the 'success_url'? The 'success_url' will redirect the user agent to
the 'returnTo' parameter provided when requesting authentication.
It's necessary to enable a silent login on any URL.

Minimal test coverage has been added for these two custom views to ensure
correct behavior.

src/backend/core/authentication/views.py

@@ -1,5 +1,6 @@
 """Authentication Views for the People core app."""
 
+import copy
 from urllib.parse import urlencode
 
 from django.contrib import auth
@@ -11,6 +12,12 @@ from django.utils import crypto
 from mozilla_django_oidc.utils import (
     absolutify,
 )
+from mozilla_django_oidc.views import (
+    OIDCAuthenticationCallbackView as MozillaOIDCAuthenticationCallbackView,
+)
+from mozilla_django_oidc.views import (
+    OIDCAuthenticationRequestView as MozillaOIDCAuthenticationRequestView,
+)
 from mozilla_django_oidc.views import (
     OIDCLogoutView as MozillaOIDCOIDCLogoutView,
 )
@@ -135,3 +142,40 @@ class OIDCLogoutCallbackView(MozillaOIDCOIDCLogoutView):
         auth.logout(request)
 
         return HttpResponseRedirect(self.redirect_url)
+
+
+class OIDCAuthenticationCallbackView(MozillaOIDCAuthenticationCallbackView):
+    """Custom callback view for handling the silent loging flow."""
+
+    @property
+    def failure_url(self):
+        """Override the failure URL property to handle silent login flow
+
+        A silent login failure (e.g., no active user session) should not be
+        considered as an authentication failure.
+        """
+        if self.request.session.get("silent", None):
+            del self.request.session["silent"]
+            self.request.session.save()
+            return self.success_url
+        return super().failure_url
+
+
+class OIDCAuthenticationRequestView(MozillaOIDCAuthenticationRequestView):
+    """Custom authentication view for handling the silent loging flow."""
+
+    def get_extra_params(self, request):
+        """Handle 'prompt' extra parameter for the silent login flow
+
+        This extra parameter is necessary to distinguish between a standard
+        authentication flow and the silent login flow.
+        """
+        extra_params = self.get_settings("OIDC_AUTH_REQUEST_EXTRA_PARAMS", None)
+        if extra_params is None:
+            extra_params = {}
+        if request.GET.get("silent") == "true":
+            extra_params = copy.deepcopy(extra_params)
+            extra_params.update({"prompt": "none"})
+            request.session["silent"] = True
+            request.session.save()
+        return extra_params