Add data for long-tail contact links

Introduce the distinction between popular and long-tail links
Add long-tail contact links to interstitial page
Extract contact link rendering to partial
GOV.UK styling for 'details' tag, JS-polyfill
Nuke unnecessary whitespace
Merge pull request #118 from alphagov/add-long-tail-contact-links-to-interstitial-page

Add long-tail contact links to interstitial page