Added user satisfaction survey

Added the user satisfaction survey. I've taken two JS files from the alpha.gov.uk codebase:
https://github.com/alphagov/static/blob/master/app/assets/javascripts/user-satisfaction-survey.js
and 
https://github.com/alphagov/govuk_template/blob/master/source/assets/javascripts/cookie-functions.js
We might need to find a better way of managing them.

For now, it's only shown on the landing page and won't be shown if you've opted out or answered it before. It sets a cookie to remember if a user has filled in the survey or opted out.