Store CRS to avoid costly lookup

Looking up the coordinate reference system for a given polygon’s bounds
is surprisingly expensive.

We can make things run faster by precomputing it at the time of
generating the simplified polygons, then storing it in the SQLite
database alongside the polygon data.

app/broadcast_areas/models.py

@@ -95,20 +95,27 @@ class BroadcastArea(BaseBroadcastArea, SortableMixin):
     @classmethod
     def from_row_with_simple_polygons(cls, row):
         instance = cls(row[:4])
-        instance.simple_polygons = Polygons(row[4])
+        instance.simple_polygons = Polygons(
+            row[4],
+            utm_crs=row[5],
+        )
         return instance
 
     @cached_property
     def polygons(self):
+        polygons, utm_crs = BroadcastAreasRepository().get_polygons_for_area(self.id)
         return Polygons(
-            BroadcastAreasRepository().get_polygons_for_area(self.id)
+            polygons, utm_crs=utm_crs
         )
 
     @cached_property
     def simple_polygons(self):
-        return Polygons(
+        simple_polygons, utm_crs = (
             BroadcastAreasRepository().get_simple_polygons_for_area(self.id)
         )
+        return Polygons(
+            simple_polygons, utm_crs=utm_crs
+        ).utm_polygons
 
     @cached_property
     def sub_areas(self):