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]/expenses/expense-card.tsx

@@ -38,7 +38,8 @@ export function ExpenseCard({ expense, currency, groupId }: Props) {
           {expense.title}
         </div>
         <div className="text-xs text-muted-foreground">
-          Paid by <strong>{expense.paidBy.name}</strong> for{' '}
+          {expense.amount > 0 ? 'Paid by ' : 'Received by '}
+          <strong>{expense.paidBy.name}</strong> for{' '}
           {expense.paidFor.map((paidFor, index) => (
             <Fragment key={index}>
               {index !== 0 && <>, </>}