Spike: Split application.scss into 4 parts

What does this MR do and why?

Science as part of gitlab-org/frontend/rfcs#92 (closed)

Add a query param no_startup_css which removes startup CSS, instead splitting application.css into 4 pieces. This is some science to determine if we can get better performance gains without the current startup CSS setup. Some prior discussion in this thread on the Epic.

The goal of this is optimizing our most frequent page views (when assets are cached), compared to our current strategy which prioritises uncached first contentful paint.

Screenshots or screen recordings

Initial measurement just a lighthouse trace before/after as I am lazy. Measured using dev build, so don't mind the huge numbers. First paint and total blocking time are longer (the latter is concerning..)

with startup	no startup css

How to set up and validate locally

1. Add ?use_startup_css=true to URL query params to test the split files and no startup_css stuff.

MR acceptance checklist

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