Add limits to max/min bleed

This prevents us from giving unrealistically large or small bleed
estimates in case we have areas which are more dense or less dense than
the most/least dense areas we currently have.

Also means we don’t have to treat City of London as a special case.

tests/app/broadcast_areas/test_broadcast_area.py

@@ -315,7 +315,7 @@ def test_phone_density(
 @pytest.mark.parametrize('area, expected_bleed_in_m, expected_bleed_in_degrees', (
     (
         # Islington (most dense in UK)
-        'lad20-E09000019', 488, 0.00439
+        'lad20-E09000019', 500, 0.00449
     ),
     (
         # Cordwainer Ward (City of London)
@@ -332,7 +332,7 @@ def test_phone_density(
     ),
     (
         # Highland (least dense in UK)
-        'lad20-S12000017', 5_095, 0.0458
+        'lad20-S12000017', 5_000, 0.0449
     ),
 ))
 def test_estimated_bleed(

tests/app/main/views/test_broadcast.py

@@ -649,7 +649,7 @@ def test_broadcast_page(
         'Islington remove',
     ], [
         'An area of 9.7 square miles Will get the alert',
-        'An extra area of 4.6 square miles is Likely to get the alert',
+        'An extra area of 4.7 square miles is Likely to get the alert',
         '200,000 to 300,000 phones',
     ]),
 ))