Adding negative debt (credit) through an activity fails #14
Description
It seems that it is not possible to assign negative debt (credit) to a person through the add activity function. When I submit a negative value (see screenshot), Tabby tells me that it has successfully added the debt (with a green redirect screen), although in the overview nothing (not positive nor negative) is actually added to the person's account.
Although I understand that this feature might not be supported, it should in the current state at least return some error as a warning. (My suspicion is that something could be missing near https://github.com/bertvandepoel/tabby/blob/master/index.php#L302 .) Personally, I would prefer it to work, rather than reporting an error, as it would be a useful feature that I could use.