Fix Sass compilation warnings

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What

Remove Sass compilation warnings, specifically:

remove percentage calculation in contents-list-with-body and replace with a simpler value (it's for the contents list element, which only ever says 'Contents' so limiting it to two thirds of the page seems odd anyway)
replace use of govuk-gutter in worldwide organisation pages (e.g. https://www.gov.uk/world/organisations/british-embassy-berlin) for the govuk-frontend spacing scale, which fits better and doesn't require any calculations
remove a font size 14 in the figure component, as recommended by new Design System min 16 font size guidance

Warnings are becoming confusing and need fixing.

None.