Format postcode for CSV letter rows

We are formatting the postcode here, because if we did it in template
preview, that could break flows like API and admin one-off, since
we are not validating postcode there yet, and format_postcode
needs a nice validated postcode.

We are not doing it in admin, as then we would have to either
rewrite the CSV file or pass data differently to API. First would
be nasty, second is a lot of overhead.

In the long run we might want to move postcode formatting to
template preview so that the postcode in letter preview looks the same
before and after user sends it, but now to get it out quickly it's better
to format the postcode here in the task.

tests/app/celery/test_tasks.py

@@ -961,7 +961,7 @@ def test_save_letter_saves_letter_to_database(mocker, notify_db_session):
         'addressline4': 'Wibble',
         'addressline5': 'Wobble',
         'addressline6': 'Wubble',
-        'postcode': 'Flob',
+        'postcode': 'SE1 2SA',
     }
     notification_json = _notification_json(
         template=job.template,
@@ -1021,6 +1021,31 @@ def test_save_letter_saves_letter_to_database_with_correct_postage(mocker, notif
     assert notification_db.postage == postage
 
 
+def test_save_letter_saves_letter_to_database_with_formatted_postcode(mocker, notify_db_session):
+    service = create_service(service_permissions=[LETTER_TYPE])
+    template = create_template(service=service, template_type=LETTER_TYPE)
+    letter_job = create_job(template=template)
+
+    mocker.patch('app.celery.tasks.letters_pdf_tasks.create_letters_pdf.apply_async')
+    notification_json = _notification_json(
+        template=letter_job.template,
+        to='Foo',
+        personalisation={'addressline1': 'Foo', 'addressline2': 'Bar', 'postcode': 'se1 64sa'},
+        job_id=letter_job.id,
+        row_number=1
+    )
+    notification_id = uuid.uuid4()
+    save_letter(
+        letter_job.service_id,
+        notification_id,
+        encryption.encrypt(notification_json),
+    )
+
+    notification_db = Notification.query.one()
+    assert notification_db.id == notification_id
+    assert notification_db.personalisation["postcode"] == "SE16 4SA"
+
+
 def test_save_letter_saves_letter_to_database_right_reply_to(mocker, notify_db_session):
     service = create_service()
     create_letter_contact(service=service, contact_block="Address contact", is_default=True)
@@ -1037,7 +1062,7 @@ def test_save_letter_saves_letter_to_database_right_reply_to(mocker, notify_db_s
         'addressline4': 'Wibble',
         'addressline5': 'Wobble',
         'addressline6': 'Wubble',
-        'postcode': 'Flob',
+        'postcode': 'SE1 3WS',
     }
     notification_json = _notification_json(
         template=job.template,