Add multivariate framework to interstitial page
first variant: sections
second variant: lists
third variant: search box
tweak variant css so the styles don't clash
variant testing: move variant HTML to script tags
move jquery autocomplete plugin to vendor
adjust contact page variant 1 based on design feedback
variant 2: change show/hide to summary based on feedback
adjust variant 3 based on design feedback
fix variant 2 and variant 3 logic
incorporate review feedback into variant test code

- indent <details> block
- add "for" attr for labels
- remove unused CSS for <details> polyfill
variant test: remove unnecessary JS variable
use GOV.UK styles for <summary>
Merge pull request #105 from alphagov/contact-page-variant-testing

Variant testing of more links on the /contact interstitial page