Construct artefact in before_filter for update action.

This will allow us to insert another filter between this one and the
action to do the url-arbiter registration for both the create and update
actions.
Integrate with url-arbiter

All calls to create/update an artefact will now register the path with
the url-arbiter first.
Merge pull request #206 from alphagov/url-arbiter_integration

Integrate with url-arbiter