Always show 4 items in the example list

If a library has lots of items then the first 3 should be shown, with
a count of how many more there are, for a total of 4 list items:
> a, b, c, and 23 more

If the library only has 4 items then all 4 should be shown, with
consistent use of conjunction and Oxford comma[1]:
> a, b, c, and d

This keeps the lengths of the examples nice and consistent.

1. We use an Oxford comma because it helps disambiguate when an area
itself has a comma or ‘and’ in it, for example ‘Armagh City, Banbridge
and Craigavon’

app/broadcast_areas/repo.py

@@ -121,7 +121,7 @@ class BroadcastAreasRepository(object):
         ),
         area_count AS (SELECT COUNT(*) AS c FROM areas),
         subset_area_count AS (SELECT c - 4 FROM area_count),
-        description_area_names  AS (SELECT name FROM areas ORDER BY name ASC LIMIT 4),
+        description_area_names  AS (SELECT name FROM areas ORDER BY name ASC LIMIT 3),
         description_areas_joined AS (
             SELECT GROUP_CONCAT(name, ", ") FROM description_area_names
         )
@@ -129,6 +129,8 @@ class BroadcastAreasRepository(object):
         CASE (SELECT * FROM subset_area_count)
         WHEN 0 THEN
             (SELECT * FROM description_areas_joined)
+            || ", and "
+            || (SELECT name from areas ORDER BY name DESC limit 1)
         ELSE
             (SELECT * FROM description_areas_joined)
             || ", and "