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/broadcast_areas/models.py

@@ -7,6 +7,8 @@ from notifications_utils.serialised_model import SerialisedModelCollection
 from rtreelib import Rect
 from werkzeug.utils import cached_property
 
+from app.formatters import square_metres_to_square_miles
+
 from .populations import CITY_OF_LONDON
 from .repo import BroadcastAreasRepository, rtree_index
 
@@ -55,13 +57,13 @@ class BaseBroadcastArea(ABC):
 
     @cached_property
     def simple_polygons_with_bleed(self):
-        return self.simple_polygons.bleed_by(self.estimated_bleed_in_degrees)
+        return self.simple_polygons.bleed_by(self.estimated_bleed_in_m)
 
     @cached_property
     def phone_density(self):
         if not self.polygons.estimated_area:
             return 0
-        return self.count_of_phones / self.polygons.estimated_area
+        return self.count_of_phones / square_metres_to_square_miles(self.polygons.estimated_area)
 
     @property
     def estimated_bleed_in_m(self):
@@ -72,14 +74,10 @@ class BaseBroadcastArea(ABC):
         range masts, so the typical bleed will be high (up to 5,000m).
         '''
         if self.phone_density < 1:
-            return Polygons.approx_bleed_in_degrees * Polygons.approx_metres_to_degree
+            return Polygons.approx_bleed_in_m
         estimated_bleed = 5_900 - (math.log(self.phone_density, 10) * 1_250)
         return max(500, min(estimated_bleed, 5000))
 
-    @property
-    def estimated_bleed_in_degrees(self):
-        return self.estimated_bleed_in_m / Polygons.approx_metres_to_degree
-
 
 class BroadcastArea(BaseBroadcastArea, SortableMixin):
 
@@ -123,7 +121,7 @@ class BroadcastArea(BaseBroadcastArea, SortableMixin):
     def count_of_phones(self):
         if self.id.endswith(CITY_OF_LONDON.WARDS):
             return CITY_OF_LONDON.DAYTIME_POPULATION * (
-                self.polygons.estimated_area / CITY_OF_LONDON.AREA_SQUARE_MILES
+                self.polygons.estimated_area / CITY_OF_LONDON.AREA_SQUARE_METRES
             )
         if self.sub_areas:
             return sum(area.count_of_phones for area in self.sub_areas)
@@ -169,7 +167,10 @@ class CustomBroadcastArea(BaseBroadcastArea):
         return Polygons(
             # Polygons in the DB are stored with the coordinate pair
             # order flipped – this flips them back again
-            Polygons(self._polygons).as_coordinate_pairs_lat_long
+            [
+                [[lat, long] for long, lat in polygon]
+                for polygon in self._polygons
+            ]
         )
 
     simple_polygons = polygons