In the update_letter_notifications_statuses task we now check whether
each row in the response file that we receive from the DVLA has the
value of 'Sorted' or 'Unsorted' in the postcode validation field. We
then calculate the number of Sorted and Unsorted rows for each day and
save each day as a row in the daily_sorted_letter table.
The data in daily_sorted_letter table should be in local time, so we
convert the datetime before saving.