Add function to estimate letter delivery date

Letter delivery depends on:
- how long it takes to print
- how long it takes to post

Both of these process are impacted by weekends, because people don’t
work on weekends.

It also depends on if you submit your letter before or after 5pm,
because that’s the cut off time for getting a letter printed on a given
day – ie after 5pm on Monday is effectively the same as Tuesday and so
on.

But I reckon all our users need to know is roughly how long it will take
until the letter turns up on the user’s doorstep. So this commit adds
a function to calculate this. Doesn’t surface it on the front end _yet_.

tests/app/test_utils.py

@@ -11,7 +11,8 @@ from app.utils import (
     generate_notifications_csv,
     generate_previous_dict,
     generate_next_dict,
-    Spreadsheet
+    Spreadsheet,
+    get_estimated_delivery_date_for_letters,
 )
 
 from tests import notification_json, single_notification_json
@@ -154,3 +155,45 @@ def test_generate_notifications_csv_calls_twice_if_next_link(mocker):
     # mock_calls[0][2] is the kwargs from first call
     assert mock_get_notifications.mock_calls[0][2]['page'] == 1
     assert mock_get_notifications.mock_calls[1][2]['page'] == 2
+
+
+@pytest.mark.parametrize('upload_time, expected_estimate', [
+
+    # BST
+    # ==================================================================
+    #  First thing Monday
+    ('2017-07-10 00:00:01', ('Thursday', 'Friday')),
+    #  Monday at 16:59 BST
+    ('2017-07-10 15:59:59', ('Thursday', 'Friday')),
+    #  Monday at 17:00 BST
+    ('2017-07-10 16:00:01', ('Friday', 'Saturday')),
+    #  Tuesday before 17:00 BST
+    ('2017-07-11 12:00:00', ('Friday', 'Saturday')),
+    #  Wednesday before 17:00 BST
+    ('2017-07-12 12:00:00', ('Saturday', 'Monday')),
+    #  Thursday before 17:00 BST
+    ('2017-07-13 12:00:00', ('Monday', 'Tuesday')),
+    #  Friday anytime
+    ('2017-07-14 00:00:00', ('Wednesday', 'Thursday')),
+    ('2017-07-14 12:00:00', ('Wednesday', 'Thursday')),
+    ('2017-07-14 22:00:00', ('Wednesday', 'Thursday')),
+    #  Saturday anytime
+    ('2017-07-14 12:00:00', ('Wednesday', 'Thursday')),
+    #  Sunday before 1700 BST
+    ('2017-07-15 15:59:59', ('Wednesday', 'Thursday')),
+    #  Sunday after 17:00 BST
+    ('2017-07-16 16:00:01', ('Thursday', 'Friday')),
+
+    # GMT
+    # ==================================================================
+    #  Monday at 16:59 GMT
+    ('2017-01-02 16:59:59', ('Thursday', 'Friday')),
+    #  Monday at 17:00 GMT
+    ('2017-01-02 17:00:01', ('Friday', 'Saturday')),
+
+])
+def test_get_estimated_delivery_date_for_letter(upload_time, expected_estimate):
+    assert tuple(
+        day.strftime('%A')
+        for day in get_estimated_delivery_date_for_letters(upload_time)
+    ) == expected_estimate