Add a tour for users new to broadcast services

This is an initial, prototype-quality attempt at introducing some kind
of tour for users new to broadcasting. A lot of the users we’re speaking
to don’t have a good concept of what broadcasting means, which is
causing usability problems down the line.

We did a similar thing in the early days of Notify to explain the
concept of message templates and personalisation.

tests/app/main/views/test_verify.py

@@ -178,6 +178,7 @@ def test_activate_user_redirects_to_service_dashboard_if_user_already_belongs_to
     sample_invite,
     api_user_active,
     mock_login,
+    mock_get_service,
 ):
     mocker.patch('app.user_api_client.add_user_to_service', side_effect=HTTPError(
         response=Mock(