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.

app/models/broadcast_message.py

@@ -6,7 +6,10 @@ from notifications_utils.template import BroadcastPreviewTemplate
 from orderedset import OrderedSet
 from werkzeug.utils import cached_property
 
-from app.broadcast_areas import CustomBroadcastAreas, broadcast_area_libraries
+from app.broadcast_areas.models import (
+    CustomBroadcastAreas,
+    broadcast_area_libraries,
+)
 from app.formatters import round_to_significant_figures
 from app.models import JSONModel, ModelList
 from app.models.user import User

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,