WW API page and id URLs should use request hostname

This fixes that, so that if you start requesting e.g. whitehall-admin.preview.alphagov.co.uk, the next_page link is also on whitehall-admin.preview.alphagov.co.uk etc.
Merge pull request #519 from alphagov/fix_ww_api_page_links

Worldwide API page and Id URLs should use request hostname

https://www.pivotaltracker.com/story/show/48396379