Support accepting return URLs or email address as part of invoice

Email address:
1. Can be entered by staff at invoice creation, and then used to email the receipt

Return URL:
1. Will be used in later scenarios where a CHEFS user creates an invoice and they want to pay via CC then go back to their app

Start by locating the invoice creation, receipt-email, and payment return entry points in the repository, then inspect the existing invoice data model and API tests. Confirm how staff-entered email addresses and payment return URLs are represented. Done means staff can provide an email for receipt delivery and a CHEFS user returns to the supplied app after paying by credit card.

Written by the indexing model from the issue text.