See mockup at https://opendataservices.github.io/360-cookies/popup.html#
TODO:
- Make the No button drop a cookie so that we don't ask again
- Make the popup only appear if we don't know a user's consent status - ie, we can't see any of: our own opt-out cookie, a piwik_ignore cookie, or a _pk_id cookie (e.g. _pk_id.19.da65)