Revert "feat!: Remove django-admin default login. (#29416)" (#29824)

This reverts commit be2a57902f.

openedx/core/djangoapps/user_authn/views/login.py

@@ -11,6 +11,7 @@ import re
 import urllib
 
 from django.conf import settings
+from django.contrib import admin
 from django.contrib.auth import authenticate, get_user_model
 from django.contrib.auth import login as django_login
 from django.contrib.auth.decorators import login_required
@@ -643,9 +644,13 @@ def login_refresh(request):  # lint-amnesty, pylint: disable=missing-function-do
 
 def redirect_to_lms_login(request):
     """
-    This view redirect the admin/login url to the site's login page.
+    This view redirect the admin/login url to the site's login page if
+    waffle switch is on otherwise returns the admin site's login view.
     """
-    return redirect('/login?next=/admin')
+    if ENABLE_LOGIN_USING_THIRDPARTY_AUTH_ONLY.is_enabled():
+        return redirect('/login?next=/admin')
+    else:
+        return admin.site.login(request)
 
 
 class LoginSessionView(APIView):

openedx/core/tests/test_admin_view.py

@@ -6,8 +6,11 @@ This is not inside a django app because it is a global property of the system.
 
 from django.test import Client, TestCase
 from django.urls import reverse
+from edx_toggles.toggles.testutils import override_waffle_switch
 from common.djangoapps.student.tests.factories import UserFactory, TEST_PASSWORD
 
+from openedx.core.djangoapps.user_authn.views.login import ENABLE_LOGIN_USING_THIRDPARTY_AUTH_ONLY
+
 
 class TestAdminView(TestCase):
     """
@@ -36,7 +39,10 @@ class TestAdminView(TestCase):
         assert response.status_code == 302
 
     def test_admin_login_redirect(self):
-        """Admin login will redirect towards the site login page."""
-        response = self.client.get(reverse('admin:login'))
-        assert response.url == '/login?next=/admin'
-        assert response.status_code == 302
+        with override_waffle_switch(ENABLE_LOGIN_USING_THIRDPARTY_AUTH_ONLY, True):
+            response = self.client.get(reverse('admin:login'))
+            assert response.url == '/login?next=/admin'
+            assert response.status_code == 302
+        with override_waffle_switch(ENABLE_LOGIN_USING_THIRDPARTY_AUTH_ONLY, False):
+            response = self.client.get(reverse('admin:login'))
+            assert response.template_name == ['admin/login.html']