Contact Form: Show a Masterbar bell notification when a new Contact Form submission is made on a site

### What

The Contact Form could offer the option to show a new form submission notification in the Masterbar/Bell notifications.

### Why

This would remove the need to add an email to a Contact Form, and users could check new form submissions simply by jumping through their notifications as if they were comments.

Related to this interaction:
8239506-zd-a8c

### How

By displaying a notification every time a new form submission is made.

The issue names Contact Form submissions and Masterbar/Bell notifications but no files or tests. Start by locating the submission-handling and notification entry points, then clarify the required behavior. Done means a new submission consistently creates a notification that users can reach through the Masterbar/Bell.

Written by the indexing model from the issue text.