Add support for group income (= negative expenses) (#158)

* Allow negative amount for expenses to be entered

- an expense becomes an income
- this does not affect calculations, i.e. an income can be split just like an expense

* Incomes should not be reimbursements

when entering a negative number
- deselect 'isReimbursement'
- hide reimbursement checkbox

* Change captions when entering a negative number

- "expense" becomes "income"
- "paid" becomes "received"

* Format incomes on expense list

- replace "paid by" with "received by"

* Format incomes on "Stats" tab

- a group's or participants balance might be negative
- in this case "spendings" will be "earnings" (display accordingly)
- always display positive numbers
- for active user: highlight spendings/earnings in red/green

* Fix typo

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Co-authored-by: Sebastien Castiel <sebastien@castiel.me>

src/app/groups/[groupId]/stats/totals-group-spending.tsx

@@ -6,11 +6,12 @@ type Props = {
 }
 
 export function TotalsGroupSpending({ totalGroupSpendings, currency }: Props) {
+  const balance = totalGroupSpendings < 0 ? 'earnings' : 'spendings'
   return (
     <div>
-      <div className="text-muted-foreground">Total group spendings</div>
+      <div className="text-muted-foreground">Total group {balance}</div>
       <div className="text-lg">
-        {formatCurrency(currency, totalGroupSpendings)}
+        {formatCurrency(currency, Math.abs(totalGroupSpendings))}
       </div>
     </div>
   )