Remove false precision from area estimates

We give estimates of the area for those who can’t see the map. These
estimates were needlessly precise, gave a false sense of accuracy and
were causing intermittent test failures between different environments.

This commit rounds them in the same way that we round the count of
phones.

app/__init__.py

@@ -72,6 +72,7 @@ from app.formatters import (
     nl2br,
     recipient_count,
     recipient_count_label,
+    round_to_significant_figures,
     valid_phone_number,
 )
 from app.models.organisation import Organisation
@@ -564,6 +565,7 @@ def add_template_filters(application):
         iteration_count,
         recipient_count,
         recipient_count_label,
+        round_to_significant_figures,
         message_count_label,
         message_count,
         message_count_noun,

app/templates/views/broadcast/macros/area-map.html

@@ -3,7 +3,7 @@
   <ul>
     <li class="area-list-key area-list-key--certain">
       <span class="visually-hidden">
-        An area of {{ "{:,.1f}".format(broadcast_message.simple_polygons.estimated_area) }} square miles
+        An area of {{ (broadcast_message.simple_polygons.estimated_area)|round_to_significant_figures(1)|format_thousands }} square miles
       </span>
       Will get
       <span class="visually-hidden">the</span>
@@ -11,7 +11,7 @@
     </li>
     <li class="area-list-key area-list-key--likely">
       <span class="visually-hidden">
-        An extra area of {{ "{:,.1f}".format(broadcast_message.simple_polygons_with_bleed.estimated_area - broadcast_message.simple_polygons.estimated_area) }} square miles is
+        An extra area of {{ (broadcast_message.simple_polygons_with_bleed.estimated_area - broadcast_message.simple_polygons.estimated_area)|round_to_significant_figures(1)|format_thousands }} square miles is
       </span>
       Likely to get
       <span class="visually-hidden">