[#1490] Fox datapusher_hook callback URL

The old implementation didn't work for deployments that were not located
on a non-root location (eg http://someorg.com/ckan). This fixes it and
also allows the action to be run outside of a web request eg from the command
line
Merge pull request #1490 from ckan/1490-datapusher_hook-callback-url

Wrong datapusher hook callback URL on non-root deployments