Change performance stats to be 2 decimal places

We are starting to see lots of 100.0%s in the current table
and we think this looks suspiciously too good so think it is
beneficial to change it to be 2dp such that we get a few more
non 100.0% values.

This will put all the values in to having 2dp, however it will
also require the API to have a change to
https://github.com/alphagov/notifications-api/blob/master/app/performance_dashboard/rest.py#L81
where it is currently losing the granularity down to a single
decimal place (meaning that if we were really at 99.894% then that
would be shown on the page as 99.90% rather than 99.89%). However,
I don't think it is a blocker that we get that sorted before this
can be merged.

app/templates/views/performance.html

@@ -88,7 +88,7 @@
   <div class="govuk-grid-row">
     <div class="govuk-grid-column-one-half">
       {{ big_number(
-        '{:.1f}%'.format(average_percentage_under_10_seconds),
+        '{:.2f}%'.format(average_percentage_under_10_seconds),
         label='on average',
       ) }}
     </div>
@@ -107,7 +107,7 @@
         {{ item.date | format_date_normal }}
       {% endcall %}
       {% call field() %}
-        {{ item.percentage_under_10_seconds }}%
+        {{ '{:.2f}%'.format(item.percentage_under_10_seconds) }}
       {% endcall %}
     {% endcall %}
     <p class="table-show-more-link">