Ensures proper urls are formed for replacing unpublished content

Updates PR https://github.com/alphagov/whitehall/pull/1629

Removes '/deleted-' from the urls formed by RegisterableEdition so that
causes existing artefacts in Panopticon to be updated, instead of
creating an new artefact record

These assertions would be better asserting on the whole slug.  That way we're ensuring that the regex doesn't munge the slug in other interesting ways...