Fix the hosts for organisations' web_urls in API

The organisations API correctly returned the web_url for an
organisation using the public host, but those for child and
parent organisations were incorrectly using the API host. This
commit makes all web_urls returned by the organisations API use
the public host.
Merge pull request #944 from alphagov/fix_organisations_api_parent_child_web_url_host

Fix the hosts for organisations' web_urls in API