feat: ENABLE_LEGACY_LIBRARY_MIGRATOR flag added (#2440)

- Adds the `ENABLE_LEGACY_LIBRARY_MIGRATOR` flag. **Reason:** The migrator frontend is finishing before the backend; this flag is mainly to hide it until it is fully connected and working with the backend.
- Puts the migration warning under the new flag.

.env.development

@@ -39,6 +39,7 @@ ENABLE_VIDEO_UPLOAD_PAGE_LINK_IN_CONTENT_DROPDOWN=true
 ENABLE_CERTIFICATE_PAGE=true
 ENABLE_NEW_VIDEO_UPLOAD_PAGE=true
 ENABLE_TAGGING_TAXONOMY_PAGES=true
+ENABLE_LEGACY_LIBRARY_MIGRATOR=true
 BBB_LEARN_MORE_URL=''
 HOTJAR_APP_ID=''
 HOTJAR_VERSION=6