Rename electoral wards to local areas

We’ve observed people using ‘national’ and ‘local’ during user research.
It has less tongue-twisting ambiguity than county vs country.

But we think that maybe just getting rid of ‘counties’ is enough to
disambiguate them. So this commit just takes the ‘local’ concept.

This commit also gives the libraries and areas new IDs, which means if
we want to rename them in the future it won’t be a breaking change.

tests/app/models/test_broadcast_message.py

@@ -11,9 +11,9 @@ def test_simple_polygons(fake_uuid):
         created_by_id=fake_uuid,
         areas=[
             # Hackney Central
-            'electoral-wards-of-the-united-kingdom-E05009372',
+            'wd20-E05009372',
             # Hackney Wick
-            'electoral-wards-of-the-united-kingdom-E05009374',
+            'wd20-E05009374',
         ],
     ))