PERF: Avoid N+1 queries when generating `Stylesheet::Manager.stylesheet_details` (PR #13356)

Assets are served via the server in development and the default 20 traces is too little for the number of assets we load in development.


Instead of waiting for Theme.find_by to be called on the theme_id, we eager load stuff and just assign it here.

The title of this pull request changed from “DEV: Increase number of mini-profiler traces in development.” to "PERF: Avoid N+1 queries when generating Stylesheet::Manager.stylesheet_details

This is a duplicate with another PR I think? Should rebase away.

This PR may no longer be relevant as I’m looking to restructure how we handle themes on the backend.