List Dropcontact billing invoices

Automatically list Dropcontact billing invoices on dropcontact.com. List the billing invoices of the signed-in Dropcontact account. Returns per invoice: date, amount, status, statusTone and hostedInvoiceUrl (an invoice page with its own download button, so this returns links rather than PDF files). `status` is the wording shown, in whatever language it is served; `statusTone` is the language-independent reading of the same badge ("positive" for a settled invoice). date and amount are nullable. Accounts with no active subscription have no invoices to read and say so.