BUG: group shows as settled up, even though there are balances in other currencies

[krokosik]

Repro:

• create a group and add an expense that does not involve us, in NZD
• add an expense that includes us in USD
• in /groups page, we have settled up, even though the total inside the group says otherwise

We should see the group balance, preferable using the defaultCurrency group property, but since there is no way to set or edit it as of now, I propose to show the highest debt first.