Increase contrast of focused NHS logo in header

[paulrobertlloyd]

Description

(Split out from #1000)

• Update the NHS logo in the header to use a single path, as opposed to paths for the containing rectangle and the NHS letters.
• This means the log can appear with greater contrast, inherit global focus styles, and require less CSS to style.

As this changes the HTML markup, this is a breaking change.

Before	After

Checklist

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

[paulrobertlloyd]

Urgh, no idea why tests are failing, as I fixed those issues flagged in the linter. Hmmm.