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
We're already overriding the GOV.UK template here, so you can actually just remove this whole selector.