Form Block Submission Email Receipt

### What

Addition of the ability to send confirmation emails to senders that use the form block.

### Why

It would create a better user experience for the sender of the form to get an email receipt showing that the form was successfully submitted and give a peace of mind to the site owner, knowing that the submitter knows that their message was successfully received.

### How

Provide a choice in editor where a site owner can activate this feature.

The issue does not name files, tests, or an entry point. Start by locating the form block and its editor settings, then determine how a sender email receipt and site-owner activation choice should be specified; done means the feature behavior and acceptance criteria are defined and tested.

Written by the indexing model from the issue text.