feat: Only display Preview Language Setting (dark_lang) in LMS

Previously, there were two (identical) ways to preview dark language:

1. <CMS_ROOT_URL>/update_lang
2. <LMS_ROOT_URL>/update_lang

Now, (1) will simply redirect to (2). So, the Preview Language Setting
page will only render in an LMS context. This has no impact on end-user
functionality. It has only a very minor UX end-user impact.

The purpose of this is to allow us to completely retire the legacy
Studio frontend without losing any end-user functionality. This page is
not yet available in an MFE, which will need to be the eventual
solution.

Author: kdmccormick

cms/urls.py

@@ -9,6 +9,7 @@
 from django.urls import path, re_path
 from django.utils.translation import gettext_lazy as _
 from django.contrib import admin
+from django.shortcuts import redirect
 from drf_spectacular.views import SpectacularAPIView, SpectacularSwaggerView
 from auth_backends.urls import oauth2_urlpatterns
 from edx_api_doc_tools import make_docs_urls
@@ -87,7 +88,7 @@
          ),
 
     # Darklang View to change the preview language (or dark language)
-    path('update_lang/', include('openedx.core.djangoapps.dark_lang.urls', namespace='dark_lang')),
+    path('update_lang/', lambda request: redirect(f'{settings.LMS_ROOT_URL}/update_lang/')),
 
     # For redirecting to help pages.
     path('help_token/', include('help_tokens.urls')),

openedx/core/djangoapps/dark_lang/tests.py

@@ -251,6 +251,9 @@ def _post_set_preview_lang(self, preview_language):
         """
         Sends a post request to set the preview language
         """
+        # @@TODO make this call set_user_preference,
+        # and then have a small separate LMS-only test class just to call the
+        # POST and ensure it sets the user preference.
         return self.client.post('/update_lang/', {'preview_language': preview_language, 'action': 'set_preview_language'})  # lint-amnesty, pylint: disable=line-too-long
 
     def _post_clear_preview_lang(self):