Migration to clear out invalid paths and prefixes

These will all relate to the old router, and therefore should be
discarded as they don't make sense with the new router.
rake task to re-register all artefacts with router.
Bump content models to get Artefact paths validation

Also update the cleanup migration to use the validaiton helper method in
the model instead of duplicating it in the migration.
Merge pull request #95 from alphagov/fixup_paths_prefixes

Fixup Artefact paths and prefixes