Contact Form: Show the user when a contact form submission was last viewed by the site owner.

#### Is your feature request related to a problem? Please describe.
When I reach a contact form on a website, I am always suspicious that the site owner will not receive the form submission, whether it's because it sends to an email address that is not monitored, or it was not configured correctly, etc. I would like some reassurance that my contact form submission will not be in vain.

#### Describe the solution you'd like
Allow site owners to add a field to the form that displays when they last received *and viewed* a contact form submission. For example, at the top of the form, it could just say "XXX people have used this form to contact Site Owner. Why, just __4 HOURS AGO__ Site Owner replied to/viewed a contact form submission from a person just like you!"

This field could auto-hide if the interval is longer than a week or so so that it's doesn't turn into a negative indicator if the site doesn't get many contact form submissions. You wouldn't want to see "Site Owner last viewed a form submission __9 MONTHS AGO__" if the site only gets one feedback every 9 months.

Ideally, it would track when the site owner last viewed Feedback in wp-admin or viewed an email generated by the contact form.

No implementation files or tests are named. Start by tracing how contact form submissions are received and recorded, then review the wp-admin Feedback and contact-form email flows mentioned in the issue. Done should include a site-owner-configurable field showing the last received-and-viewed submission, with the proposed age-based hiding behavior resolved.

Written by the indexing model from the issue text.