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/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',
     ]),
 ))