🔧(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/frontend/apps/e2e/__tests__/app-impress/config.spec.ts

@@ -9,6 +9,7 @@ const config = {
   CRISP_WEBSITE_ID: null,
   COLLABORATION_WS_URL: 'ws://localhost:4444/collaboration/ws/',
   ENVIRONMENT: 'development',
+  FRONTEND_CSS_URL: null,
   FRONTEND_THEME: 'default',
   MEDIA_BASE_URL: 'http://localhost:8083',
   LANGUAGES: [