Split utilities stylesheet to standalone file

[aduth]

🛠 Summary of changes

Splits utilities out of application.css to its own standalone stylesheet utilities.css.

Why?

• Predictability. There have been a number of issues ([1] [2]) related to the load order of stylesheets, and particularly utility classes and the relative ordering of the design system styles and our application-specific styles. This new implementation ensures that the general priority ordering of utilities > application styles > design system styles takes effect.
• Performance. While there's not expected to be a net reduction in overall page size, splitting the stylesheets into smaller chunks allows parallelized HTTP/2 downloads to finish page load sooner than a single monolithic stylesheet. The current sign-in page asset sizes are primed for this split, where as of today https://secure.login.gov application is the largest first-party asset at 22.4kb (brotli'd), with the second-largest being 15.7kb (brotli'd). These changes bring the largest asset more into equilibrium, with application.css at 10.7kb (brotli'd) and utilities.css at 8.1kb (brotli'd).

📜 Testing Plan

Verify no regressions in the display of components affected by stylesheets, particularly around precedence of application styles vs. design system styles, and vs. utility classes. Refer to [1] [2] referenced above for examples of edge-cases previously encountered.