Implement pa11y in Page Kit

[magsallen]

Implement pa11y in Page Kit and integrate pa11y checks in CI

Some useful links:
pa11y/pa11y
Financial-Times/n-ui:components/n-ui/header/pa11y-config.js@a84d0d3
Financial-Times/n-ui:components/n-ui/header/template.html@7a4e104#L1-L4