use GOVUK_WEBSITE_ROOT instead of Plek#find('www')

From Plek 1.0, Plek can no longer return URLs which aren't of the form
<app>.$GOVUK_APP_DOMAIN. Plek.current.find('www') is used by panopticon
to generate user-visible URLs - ie in prod it needs to be
https://www.gov.uk, while in preview it needs to be
https://www.preview.alphagov.co.uk.

The environment variable GOVUK_WEBSITE_ROOT is now provided to fill the
role that Plek.current.find('www') used to play.