Exclude closed orgs from screenshots

If they are closed, they probably don't have a website.
Update offsite organisation screenshots

1. The three devolved administrations were renamed and had their slugs
changed so the existing screenshot files no longer map.
2. A number of screenshots exist for orgs that no longer exist or have
had their slug changed.
2. A number of new organisations have been added which we do not have
screenshots for.
Merge pull request #1369 from alphagov/add-missing-organisation-screenshots

Add missing organisation screenshots