Provide method to uncancel / reactivate a cancelled subscription

Problem/Motivation

There are circumstances when a subscription is cancelled owing to a failed payment, then the payment goes through at the payment gateway after a delay. The situation arose as described in 🐛 Prevent an attempt to use an expired card Active .

The successful pyament does not start a new subscription. Subsequent cleaning up of the situation, short of refunding the customer and asking them to take a new sub, is a problem.

The problem could be fixed by providing a function, and maybe a UI button, to reactivate a cancelled subscription.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes