Add letter class row to settings page, visible to platform admin

Added a new row to the settings table, 'Post class', which shows the
default letter class of a service and is only visible to Platform Admin.

Also added a new page to enable Platform Admin users to change the
default letter class for a service - this only has two options at the
moment, 1st class only and 2nd class only.

tests/__init__.py

@@ -139,6 +139,7 @@ def service_json(
     email_branding=None,
     branding='govuk',
     created_at=None,
+    letter_class='second',
     letter_contact_block=None,
     inbound_api=None,
     service_callback_api=None,
@@ -168,6 +169,7 @@ def service_json(
         'email_branding': email_branding,
         'branding': branding,
         'created_at': created_at or str(datetime.utcnow()),
+        'letter_class': letter_class,
         'letter_contact_block': letter_contact_block,
         'dvla_organisation': '001',
         'permissions': permissions,

tests/app/main/views/test_service_settings.py

@@ -89,6 +89,7 @@ def mock_get_service_settings_page_common(
 
         'Label Value Action',
         'Send letters Off Change',
+        'Post class 2nd class only Change',
 
         'Label Value Action',
         'Organisation Org 1 Change',
@@ -1849,6 +1850,38 @@ def test_set_letter_branding_saves(
     mock_update_service.assert_called_once_with(service_one['id'], dvla_organisation='500')
 
 
+def test_set_letter_class_platform_admin_only(
+    logged_in_client,
+    service_one,
+):
+    response = logged_in_client.get(url_for('main.service_set_letter_class', service_id=SERVICE_ONE_ID))
+    assert response.status_code == 403
+
+
+def test_set_letter_class_prepopulates(
+    logged_in_platform_admin_client,
+    service_one,
+):
+    response = logged_in_platform_admin_client.get(url_for('main.service_set_letter_class', service_id=SERVICE_ONE_ID))
+    assert response.status_code == 200
+    page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
+    assert page.select('input[checked]')[0]['value'] == 'second'
+
+
+def test_set_letter_class_saves(
+    logged_in_platform_admin_client,
+    service_one,
+    mock_update_service,
+):
+    response = logged_in_platform_admin_client.post(
+        url_for('main.service_set_letter_class', service_id=SERVICE_ONE_ID),
+        data={'letter_class': 'first'}
+    )
+    assert response.status_code == 302
+    assert response.location == url_for('main.service_settings', service_id=SERVICE_ONE_ID, _external=True)
+    mock_update_service.assert_called_once_with(SERVICE_ONE_ID, letter_class='first')
+
+
 @pytest.mark.parametrize('current_branding, expected_values, expected_labels', [
     (None, [
         'None', '1', '2', '3', '4', '5',