Top-up of negative wallets

[dasGoogle]

We cannot top up a negative wallet when the resulting value is still negative (i.e., a -20 € wallet cannot be topped up by 10 €).

Discuss: Is that okay? If not, do we build an additional view for wallet management?

[BenBals]

Hmm, that's odd. I'd prefer that to be the case if that's not too much hassle to fix. (I don't think so because, in #12, I already factored out the "arbitrary wallet transaction" method which Julian built. We're probably able to use that for this api route instead.)