Add billing dates to subscriptions

[thinkverse]

Currently the next and last billing dates aren't added to the subscriptions. There should be a next_bill_date attribute, but that isn't in the migration and is not set when a new subscription is added. This PR resolves this issue by adding two new attributes to the subscriptions, namely the last_payment_at and next_payment_at fields.

With those, we can track when the last payment was made and when to expect the next one. Also included a fix for cancelled_at which was never set when a subscription was canceled.