Add method to get wards for custom polygon

This will be used as part of the area aggregation in the following
commits.

app/broadcast_areas/models.py

@@ -162,6 +162,13 @@ class CustomBroadcastArea(BaseBroadcastArea):
 
     simple_polygons = polygons
 
+    @cached_property
+    def overlapping_electoral_wards(self):
+        return [
+            area for area in self.nearby_electoral_wards
+            if area.simple_polygons.intersects(self.polygons)
+        ]
+
     @cached_property
     def nearby_electoral_wards(self):
         if not self.polygons:

tests/app/broadcast_areas/custom_polygons.py

@@ -13,3 +13,11 @@ SKYE = [
     [57.7334, -6.8280],
     [57.1004, -6.8280],
 ]
+
+SANTA_A = [
+    [25.8890, 66.5500],
+    [25.8890, 66.551],
+    [25.8910, 66.551],
+    [25.8910, 66.5500],
+    [25.889, 66.55000],
+]

tests/app/broadcast_areas/test_models.py

@@ -0,0 +1,20 @@
+import pytest
+
+from app.broadcast_areas.models import CustomBroadcastArea
+from tests.app.broadcast_areas.custom_polygons import BRISTOL, SANTA_A, SKYE
+
+
+@pytest.mark.parametrize(('simple_polygon', 'expected_wards_length'), [
+    (SKYE, 2),
+    (BRISTOL, 12),
+    (SANTA_A, 0)  # does not overlap with UK
+])
+def test_custom_broadcast_area_overlapping_electoral_wards(
+    simple_polygon,
+    expected_wards_length,
+):
+    custom_area = CustomBroadcastArea(
+        name='foo', polygons=[simple_polygon]
+    )
+
+    assert len(custom_area.overlapping_electoral_wards) == expected_wards_length