Default API_URL to SITE_URL if missing

Otherwise file uploads and other calls where broken when using CKAN in
a non-root location.
Merge branch 'master' into 960-dont-use-api_url
[#960] Remove ckan.api_url completely on 2.2
Merge pull request #960 from okfn/960-dont-use-api_url

api_url needs to default to CKAN's mount URL.