Updated UK subdivisions list

 These choices will not please everyone, but I once spent a long time
 weighing all the options, and these seemed to please the majority:

- Metropolitan and non-metropolitan counties of England (not the
  ceremonial counties)
- Principal areas of Wales (not the preserved or historic counties)
- Counties of Northern Ireland (not the more recent, but less
  well-known, districts of Northern Ireland)
- Council areas of Scotland

The only change I've made from the linked sources was to rename
"Greater London" to "London".

This follows on from discussions in
https://github.com/django/django-localflavor/pull/43