fix various accessibility issues

[maxstarkenburg]

as reported by siteimprove

[maxstarkenburg]

0af5f51 "Naming a generic section/landmark as per SiteImprove's recommendation"

[maxstarkenburg]

I'm going to go ahead and close this issue in this repo.

• The first commit will be covered by a more specific ticket in the new repo.
• Commits 2 and 5 were based on the old non-Primo dropdowns.
• The 3rd commit about generic landmarks should probably instead follow whatever convention GW Drupal implements.
• The 4th commit would probably be better done by conditionally adding an sr-only class to the homepage's h1 instead of a new class added just to homepage.css, though however it's handled, it should also have its own ticket in the new repo.