Create a bill by uploading an invoice

An empty draft bill is created. To upload the invoice, submit a multipart/form-data POST to invoiceUpload.url: add each key/value from invoiceUpload.fields as a form field, then append the file last. Subsequently, the invoice will be parsed and the bill is prefilled with suggestions based on the invoice. The status of parsing can be monitored by either listening for bill.documentUpdated events or polling GET /v1/web/bill/{billId}/document/{id}.