CSS refactor part I

Leaving the grid mixins out for now
Include default type styles in typography.scss so we can set a page of standard elements (p, h1, h2 etc) and it displays properly
Remove performance-platform-outer because it's not needed
$govuk-blue instead of PP $primary-colour because we are one GOV.UK
Reduce size of impact number on desktop for consistency with the summary figure on other graphs... and it's too big, anyway