Simplify polygons before storing them

We’re going to let people pass in fairly complex polygons, but:
- we don’t want to store massive polygons
- we don’t want to pass the CBCs massive polygons

So this commit adds a step to simplify the polygons before storing them.

We think it’s best for us to do this because:
- writing code to do polygon simplification is non-trivial, and we don’t
  want to make all potential integrators do it
- the simplification we’ve developed is domain-specific to emergency
  alerting, so should throw away less information than

There’s a bit more detail about how we simplify polygons in
https://github.com/alphagov/notifications-admin/pull/3590/files

app/v2/broadcast/post_broadcast.py

@@ -1,5 +1,6 @@
 from itertools import chain
 from flask import jsonify, request
+from notifications_utils.polygons import Polygons
 from app import authenticated_service, api_user
 from app.broadcast_message.translators import cap_xml_to_dict
 from app.dao.dao_utils import dao_save_object
@@ -38,6 +39,10 @@ def create_broadcast():
 
     validate(broadcast_json, post_broadcast_schema)
 
+    polygons = Polygons(list(chain.from_iterable((
+        area['polygons'] for area in broadcast_json['areas']
+    ))))
+
     broadcast_message = BroadcastMessage(
         service_id=authenticated_service.id,
         content=broadcast_json['content'],
@@ -46,9 +51,7 @@ def create_broadcast():
             'areas': [
                 area['name'] for area in broadcast_json['areas']
             ],
-            'simple_polygons': list(chain.from_iterable((
-                area['polygons'] for area in broadcast_json['areas']
-            )))
+            'simple_polygons': polygons.smooth.simplify.as_coordinate_pairs_long_lat,
         },
         status=BroadcastStatusType.PENDING_APPROVAL,
         api_key_id=api_user.id,