Use CustomBroadcastArea to estimate phones in bleed area

Our current assumption is that the bleed area has the same population
density as the broadcast area.

This is particularly naïve when:
- the bleed area overlaps the sea – no-one lives in the sea
- the broadcast area is a village and the bleed area is the surrounding
  countryside
- the broadcast area is adjacent to a densely populated area like a city

We can be smarter about this now that we have a way of determining the
number of phones in an arbitrary area, based on the known areas that we
have population data about.

Calculating the population in an overlap is a slightly more intensive
calculation. So we only doing it for areas which are smaller enough that
it doesn’t slow things down too much. For larger areas we still use the
more naïve algorithm.

tests/app/main/views/test_broadcast.py

@@ -830,7 +830,7 @@ def test_broadcast_page(
     ], [
         'An area of 6.3 square miles Will get the alert',
         'An extra area of 22.6 square miles is Likely to get the alert',
-        '9,000 to 40,000 phones',
+        '9,000 to 10,000 phones',
     ]),
     ([
         'lad20-E09000019',
@@ -839,7 +839,26 @@ def test_broadcast_page(
     ], [
         'An area of 9.7 square miles Will get the alert',
         'An extra area of 4.7 square miles is Likely to get the alert',
-        '200,000 to 300,000 phones',
+        '200,000 to 500,000 phones',
+    ]),
+    ([
+        'ctyua19-E10000019',
+    ], [
+        'Lincolnshire remove',
+    ], [
+        'An area of 3,986.6 square miles Will get the alert',
+        'An extra area of 599.4 square miles is Likely to get the alert',
+        '500,000 to 600,000 phones',
+    ]),
+    ([
+        'ctyua19-E10000019',
+        'ctyua19-E10000023'
+    ], [
+        'Lincolnshire remove', 'North Yorkshire remove',
+    ], [
+        'An area of 9,776.2 square miles Will get the alert',
+        'An extra area of 1,654.6 square miles is Likely to get the alert',
+        '1,000,000 phones estimated',
     ]),
 ))
 def test_preview_broadcast_areas_page(
@@ -907,7 +926,7 @@ def test_preview_broadcast_areas_page(
         [
             'An area of 3,205.0 square miles Will get the alert',
             'An extra area of 763.4 square miles is Likely to get the alert',
-            '4,000 to 5,000 phones',
+            '4,000 phones estimated',
         ]
     ),
 ))