Change slug for charity commission.

Data migration to update the organisation slug, and associated data,
create a redirect and reindex documents in search.

This also renames the org screenshot file so that it can continue to be found.

**Review only please.** This will need to be merged in a co-ordinated fashion with all the other places the slug has to be updated.
This code is fairly generic and will definitely be useful in the future, so would be good to keep it somewhere permanent, eg lib/data_hygiene/