Update header styles so that .nhsuk-header__search-no-nav class is no longer needed

[paulrobertlloyd]

Description

(Split out from #1000)

• Update header styles so that .nhsuk-header__search-no-nav class is no longer needed when the header contains a search field but no navigation
• Update header Nunjucks template so that it no longer repeats markup for search and navigation

This is not a breaking change 🙂

Checklist

• Tested against our testing policy (Resolution, Browser & Accessibility)
• Follows our coding standards and style guide
• CHANGELOG entry

[paulrobertlloyd]

Oooh, the backstop regression testing is super helpful! Caught a bug, and also turns out this PR fixes a few other issues:

• Corrects the bottom padding of the header when there’s search but no navigation
• Removes the navigation border that was showing when there were no navigation items

That explains some of the updated backstop images. The remaining changes are because:

• The bottom padding is now equal to the top padding
• Subtle movement of navigation items because the navigation border is now placed on the top of the navigation list, not the bottom of the header container