Ensures proper urls are formed for replacing unpublished content

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
Updates regex for registerable edition slug creation

* Changes tests to assert against full slug
Merge pull request #1648 from alphagov/reregister-unpublished-and-archived-documents

Ensures proper urls are formed for replacing unpublished content