Increase refund time window to 24 hours

[binarybaron]

I think we should increase the refund time window from 12 hours to 24 hours.

My reasoning is:

1. Being punished as an end user is very unpleasant
2. The risk is currently shifted unproportionally on the side of Bob
3. There's no downside to this as far as I can see

This will be a breaking change unless:

• We can look into Bob and Alice having a set of refund windows that are acceptable to him
• This way we can make this change backwards compatible by allowing both 12 hours as well as 24 hours (for a transitionary period)

But I'd also be fine with this just being a breaking change.