Add white background colour to Radio and Checkbox components

[robpataki]

Add white background colour to the Radio and Checkbox components as per #554.

[daniel-ac-martin]

I think this is fine as a simple workaround, but what we really need to do is understand why the discrepancy is present, and also whether we are out of sync with govuk-frontend which is really upstream from this library.

There are some occasions where we might choose to diverge from govuk-frontend, but we should do so very sparingly and know exactly why we are doing it.

[robpataki]

@daniel-ac-martin NOTGOVUK is actually sticking to the GOVUK styles, and interestingly I am not sure why the source UI components don't apply the same white background to radios and checkboxes as it does to text inputs, text areas and selects. I've found a stale Github thread (alphagov/govuk-frontend#1625), it seems from the 2019 conversations that there might be a reason why these controls have transparent background, but the then-current team wasn't sure what that reason was. Hopefully we get some clarification on this.