Allow admin to cancel or reverse any transaction

[novoselt]

Right now it is easy to refund a credit transaction. But when a user buys a wrong license and wants to refund it, editing the license to cancel it results in a smaller refund than purchasing price, especially when it is a course license for the course that has started a few days ago. Instead of fiddling with discrepancies, it would be convenient to just cancel license purchase in the sense of creating a transaction with the same amount as the purchasing price and cancelling the license immediately.

Even better - support the same reversal for license edits and subscriptions!