Refactor header component styles

[paulrobertlloyd]

Description

This PR performs a deep dive on the CSS for the header and its various components. These styles have accumulated over time, with many rules applied in multiple instances, needlessly set or unset, or not inheriting values where they could.

This PR also fixes the issue where overflowing navigation items don’t get rolled up into the dropdown menu on desktop (I’ve pulled this into a separate, non-breaking PR – #1060).

These modifier classes are no longer needed:

• .nhsuk-header__logo--only
• .nhsuk-header__transactional--logo

There are a few stylistic changes too:

• The ‘More’ button is no longer underlined (it’s not a link, it doesn’t go anywhere, so I think this makes sense)
• The padding is now equal on all sides (nhsuk-spacing(3), 16px v 20px); this saves space and uses a defined value.
• The bottom border on the drop-down menu is now darker, so that it’s not the same colour as the default page background
• On desktop, items in the drop-down navigation are no longer centred
• The font size for organisation names as been changed, with a larger minimum font size; changes in other spacing means this shouldn’t add much to the overall header height

There are 43 updated backstop images; reviewing these I think demonstrate how these changes should hopefully be for the better.

Checklist

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

[paulrobertlloyd]

Tests failing due to fix for overflowing navigation items on desktop; will address in separate PR.