Accessibility: tab though 'desktop' menu

[rfultz]

Summary

PR #4456 made it so we can fully navigate the smallest menu with the keyboard.

We should be able to navigate the larger menu with the keyboard, too.

Completion criteria

• Just as we can fully use the smallest version of the main nav with the keyboard, we should be able to do the same for the wider menu, too.
• Tab order should make sense, taking into account the 'skip nav' link at the top, nav items and their children, and the search box.
• Aural navigation should be as helpful

Additional considerations

• How's the footer menu? (new ticket?)
• We should roll these accessibility improvements over to our other sites (new tickets?)