Fix validation of licence identifiers

Currently it's not possible for a licence to use an identifier that's
ever been used by another edition.  This relaxes that so that id's are
only validated against non-archived editions.

This also makes the same change for business-support editions
Bump gem again to 4.21.0

This adds the Artefact#slug index
Merge pull request #94 from alphagov/fix_licence_validation

Fix validation of licence identifiers and add Artefact#slug index