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.

[anandamaryon1]

Nice, shaves off some bytes too 👍

I think we should hold back the breaking changes to release together with others, rather than drip feeding them, so I'll leave this open for now.

[frankieroberto]

@anandamaryon1 @paulrobertlloyd can this be closed now as it’s been included within #1058 (Header breaking changes)?