Startup.js: Include the first discussions.json request(s) in the initial Startup.js requests

Summary

In the Diffs app we currently have moved diffs_metadata.json to StartupJS.

We currently have an ongoing effort to break apart the discussions.json request: #209784 (closed)

This is aimed at moving the first request of whatever discussions.json new batch endpoint is to anticipate the triggering of the first request.

Improvements

Quicker time to (LCFP).