Allow prize money change and status change for completed challenges

[SurendraYellu]

Feature Request:

For TopGear, we have a valid case where we will need to cancel a challenge even if it is completed and also need to set the payment to 0 even on V5 (we can do it on direct, but this is not getting reflected in V5) . These are for cases where we have seen that submitter and reviewers are same, and we do not allow such challenge payments to happen.

In such cases, we want to mark them as cancelled.