Adding views to view, add and edit service data retention policies.

Only visible to a platform admin.
A service can have a custom number of days to retain the notification data for each notification type.

tests/app/main/views/service_settings/test_service_setting_permissions.py

@@ -16,6 +16,7 @@ def get_service_settings_page(
     no_reply_to_email_addresses,
     no_letter_contact_blocks,
     single_sms_sender,
+    mock_get_service_data_retention,
 ):
     client_request.login(platform_admin_user)
     return functools.partial(client_request.get, 'main.service_settings', service_id=service_one['id'])
@@ -103,6 +104,7 @@ def test_normal_user_doesnt_see_any_toggle_buttons(
     mock_get_letter_email_branding,
     mock_get_inbound_number_for_service,
     mock_get_free_sms_fragment_limit,
+    mock_get_service_data_retention
 ):
     page = client_request.get('main.service_settings', service_id=service_one['id'])
     toggles = page.find('a', {'class': 'button'})