Rename constants to populations

This is a better name for the module because it’s:
- not just constants, there’s a method in here now
- only stuff to do with populations, not other kinds of constants

app/broadcast_areas/__init__.py

@@ -1,8 +1,8 @@
 from notifications_utils.serialised_model import SerialisedModelCollection
 from werkzeug.utils import cached_property
 
-from .constants import CITY_OF_LONDON
 from .polygons import Polygons
+from .populations import CITY_OF_LONDON
 from .repo import BroadcastAreasRepository