Count recently-returned letters on the dashboard

Currently you have no way of getting to the returned letter page. This
commit adds a link to it from the dashboard, following the pattern of
the new received text messages banner.

tests/app/main/views/test_sign_out.py

@@ -31,6 +31,7 @@ def test_sign_out_user(
     mock_get_usage,
     mock_get_free_sms_fragment_limit,
     mock_get_inbound_sms_summary,
+    mock_get_returned_letter_summary_with_no_returned_letters,
 ):
     with client_request.session_transaction() as session:
         assert session.get('user_id') is not None