Return delivery estimate for letter notifications

> For get all or get one letter the response needs to be updated so that
> it looks similar to admin app.
>
> status: created|sending --> received letter
> new column: `estimated delivery date`: derived from created at date.
> (see how the admin app is doing it)
>
> NOTE:
> At the moment we only have 2 statuses for a letter created and
> sending, but we will want to have other internal statuses that make
> sense to the Notify team but not our services. When we know those
> statuses the status map will be updated at that point.

– https://www.pivotaltracker.com/story/show/150512525

This commit implements the date (not status) part of this story.

tests/app/conftest.py

@@ -568,12 +568,12 @@ def sample_notification(
 @pytest.fixture
 def sample_letter_notification(sample_letter_template):
     address = {
-        'addressline1': 'A1',
-        'addressline2': 'A2',
-        'addressline3': 'A3',
-        'addressline4': 'A4',
-        'addressline5': 'A5',
-        'addressline6': 'A6',
+        'address_line_1': 'A1',
+        'address_line_2': 'A2',
+        'address_line_3': 'A3',
+        'address_line_4': 'A4',
+        'address_line_5': 'A5',
+        'address_line_6': 'A6',
         'postcode': 'A_POST'
     }
     return create_notification(sample_letter_template, personalisation=address)