Switch the Synchrotron reporter to use information from the Subscription model to compute its data

[javawizard]

That will allow the synchrotron reporter to report instantly without having to iterate through all Stripe subscriptions via Stripe's API each time it's about to post.

It will also make #10 easier to implement since we can detect if the subscription in question already has unpaid set to true - in which case there's no need to re-notify about a failed payment.