Migration to fix a duplicated Artefact.

The duplicate was created with a leading '/' on the slug (back in march
before we'd fixed the validations on the slug).  This updates any
artefacts that relate to the invalid one to point at the correct one,
and then removes the invalid one.
Merge pull request #210 from alphagov/fix_duplicate_artefact

Migration to fix a duplicated Artefact.