Add missing organisation screenshots

This updates the rake task to generate organisation screenshots to exclude closed organisations and also updates all the screenshots we currently have in the repo, dropping dead ones and adding new ones as appropriate:

1. The three devolved administrations were renamed and had their slugs changed so the existing screenshot files no longer map. These have been generated.
2. A number of screenshots exist for organisations that no longer exist or have had their slug changed. These have been deleted.
2. A number of new organisations have been added which we do not have screenshots for. These have been generated.

Tracker: https://www.pivotaltracker.com/story/show/68203284