Fix web_urls in api responses.

https://github.com/alphagov/whitehall/pull/1563 introduced a bug whereby
api requests would return request-relative URLs in the web_url field.
This restores the original behaviour and returns public URLs (derived
from `ENV['GOVUK_WEBSITE_ROOT']`) in these responses.
Merge pull request #1574 from alphagov/fix_api_web_urls

Fix web_urls in api responses.