Rudimentary Whitehall document URLs importer
Update existing mappings if present from Whitehall
Log warnings for unknown hosts
Handle rows without Old Urls

Most of the URLs in the file will not have them.
Only process 'published' rows

The other states mean that there is nothing to redirect to, or in the case of
'superseded', that it has been replaced.

AFAICT archived/deleted items aren't included in the CSV.