Use GOVUK_WEBSITE_ROOT environment var to get public URL

With the move to Plek 1.0.0 Plek.current.find('www') ceased to give a routable
public URL. Instead apps should expect to find that in their environment.
Consolidate URL generation

There was some duplication in how we generated URLs via the UrlHelpers module methods. I've tidied
that up a little, and fixed an issue that revealed with the setting of GOVUK_WEBSITE_ROOT for
tests.
Merge pull request #66 from alphagov/fix-public-urls

Fix public urls