Move broadcast model code into an explicit module

Previously this was hidden away in an anonymous __init__.py file.
I did think about splitting the models into individual files, like
we do with the top-level models for the app. Since the models are
only imported in one place - i.e. are all used together - it didn't
seem worth the hassle, so I've kept them in one file.

tests/app/broadcast_areas/test_broadcast_area.py

@@ -3,7 +3,7 @@ from math import isclose
 import pytest
 from custom_polygons import BRISTOL, SKYE
 
-from app.broadcast_areas import (
+from app.broadcast_areas.models import (
     BroadcastAreasRepository,
     CustomBroadcastArea,
     broadcast_area_libraries,