Make calculating overlapping areas faster

By using the simplified polygons instead of the full resolutions ones
we:
- query less data from SQLite
- pass less data around
- give Shapely a less complicated shape to do its calculations on

This makes it faster to calculate how much of each electoral ward a
custom area overlaps.

For the two areas in our tests:

Place represented by custom area | Before | After
---------------------------------|--------|--------
Bristol                          | 0.07s  | 0.02s
Skye                             | 0.02s  | 0.01s

tests/app/broadcast_areas/test_broadcast_area.py

@@ -377,7 +377,7 @@ def test_estimated_bleed(
             'Stoke Bishop',
             'Windmill Hill',
         ],
-        73_496,
+        73_119,
     ),
     (
         SKYE,
@@ -387,7 +387,7 @@ def test_estimated_bleed(
             'Na Hearadh agus Ceann a Deas nan Loch',
             'Wester Ross, Strathpeffer and Lochalsh',
         ],
-        3_517,
+        3_534,
     ),
 ))
 def test_count_of_phones_for_custom_area(