feat: don't require all fields for invoice booking.

simplify field requirements and allow for a more flexible invoice
generation.

public/locales/en/invoice-header.json

@@ -3,8 +3,6 @@
   "invoice_number": "No.",
   "issued_at": "Issued at: ",
   "due_at": "Due at:",
-  "late_fee": "Late fee:",
-  "add_late_fee": "Add late fee",
   "modal_issued_at_title": "Issued at",
   "modal_due_at_title": "Due at"
 }