🔧(backend) add FRONTEND_CSS_URL env var

We added the `FRONTEND_CSS_URL` environment
variable. It will give the possibility to add a
css layer at runtime.

src/backend/core/api/viewsets.py

@@ -1689,6 +1689,7 @@ class ConfigView(drf.views.APIView):
             "CRISP_WEBSITE_ID",
             "ENVIRONMENT",
             "FRONTEND_THEME",
+            "FRONTEND_CSS_URL",
             "MEDIA_BASE_URL",
             "POSTHOG_KEY",
             "LANGUAGES",