Fix registration of periodic tasks

According to https://docs.celeryproject.org/en/stable/userguide/periodic-tasks.html?highlight=periodic#entries, since we are adding periodic tasks in a callback in a task module, we need to use the on_after_finalize signal, rather than on_after_configure.

I also had to make some small changes to the app finalization that I worked out by trial and error.

Fixes #363 (closed).