Make estimated phone count clearer

We’ve had some feedback from user research that difference between
‘will get alert’ and ‘likely to get alert’ is not clear, and it’s hard
to tell if the latter is inclusive of the former. This leads people to
question the validity of these numbers, which is important, because an
the estimate should give you some idea of the impact of what you’re
about to do.

This commit reformats the number as a range, for example 1,000 to 2,000
phones.

If the range is small, eg 40,000,000 to 40,800,000 then this suggests
a false level of accuracy. So instead we just give one number and say
it’s an estimate, eg ‘40,000,000 phones estimated’

app/models/broadcast_message.py

@@ -128,7 +128,7 @@ class BroadcastMessage(JSONModel):
     def count_of_phones(self):
         return round_to_significant_figures(
             sum(area.count_of_phones for area in self.areas),
-            2
+            1
         )
 
     @property
@@ -136,7 +136,7 @@ class BroadcastMessage(JSONModel):
         area_estimate = self.simple_polygons.estimated_area
         bleed_area_estimate = self.simple_polygons.bleed.estimated_area - area_estimate
         return round_to_significant_figures(
-            self.count_of_phones * bleed_area_estimate / area_estimate,
+            self.count_of_phones + (self.count_of_phones * bleed_area_estimate / area_estimate),
             1
         )