Update utils to do linear transformation of polygons

Brings in https://github.com/alphagov/notifications-utils/pull/889/files

At the moment, we are not doing any transformation of features before
applying geometric algorithms to them. This is, in effect, assuming that
the earth is flat.

This new version of utils implements the transformation of our polygons
to a Cartesian plane. In other words, it converts them from being
defined in spherical degrees to metres.

For the admin app this means we need to convert places where the code
expects things to be measured in degrees to work in metres instead.

app/models/broadcast_message.py

@@ -221,7 +221,8 @@ class BroadcastMessage(JSONModel):
         polygons = Polygons(
             list(itertools.chain(*(
                 getattr(area, area_attribute) for area in self.areas
-            )))
+            ))),
+            utm_crs=self.areas[0].polygons.utm_polygons.utm_crs,
         )
         if area_attribute != 'polygons' and len(self.areas) > 1:
             # We’re combining simplified polygons from multiple areas so we