[13.x] Implement customer balances

[driesvints]

This PR implements some API's around customer balances. This will make it easier for apps to retrieve a customer's balance, apply new balance transactions and display a history of balance adjustments.

// Retrieve current balance...
$balance = $user->balance(); // $5.00

// Credit new balance...
$user->applyBalance(-500, 'Premium customer top-up.');

// Debit balance...
$user->applyBalance(300, 'Bad usage penalty.');

// Retrieve all transactions...
$transactions = $user->balanceTransactions();

// Display all transactions...
foreach ($transactions as $transaction) {
    // Specific amount of the transaction...
    $amount = $transaction->amount(); // $2.31

    // Retrieve the related invoice when available...
    $invoice = $transaction->invoice();
}

Closes #762