Don't use old data when calculating donation amounts

[Powersource]

Since we won't need that data again. As we've discussed before, if people want to do actual tracking they should use Activitywatch.

[ErikBjare]

Since the thanks are timestamped, they could be kept and simply be filtered away if they are older than the last donation to a given creator when considering future donations.

I think users will find it confusing to see that all their data disappears, so we probably have to be clear and explicit about it.