[Refresh] Progressively enhance navigation (Fixes #15360)

[alexgibson]

One-line summary

This is a bit of a refactor of the navigation CSS to try and help ensure it's accessible even without JS. This helps make our navigation hopefully more resilient in the case of JS breakage / errors in older browsers.

Issue / Bugzilla link

#15360

Testing

• Nav should function as normal on both desktop and mobile when JS is enabled.
• When JS is disabled, nav should still work on hover using desktop.
• When JS is disabled, nav should still be keyboard accessible on desktop.
• When JS is disabled, nav links should still be displayed on mobile.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.68%. Comparing base (8ca29f3) to head (b64b558).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15381   +/-   ##
=======================================
  Coverage   78.68%   78.68%           
=======================================
  Files         156      156           
  Lines        8171     8171           
=======================================
  Hits         6429     6429           
  Misses       1742     1742           

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