Explore possibility of designating shared inbox to receive notifications on submission

## Description

People would like to set up notifications to shared email inbox, not necessarily to have an account there but to make that email address aware that there is something to action in GC Forms

Currently only account holders can be added to receive notifications:

## User story
As a form administrator
I need my team to know when we receive new responses
So that we can coordinate who retrieves them from GC Forms

## From clients

> I was trying to set up my form so that **a shared mailbox can receive notifications when user have submitted a form.** How would I go about doing that?

No files, tests, or entry points are named in the issue. Start by tracing how submission notifications are configured and delivered, then determine whether shared inboxes can be supported without an account; done means documenting or implementing a decided approach and verifying that the team can receive new-response notifications.

Written by the indexing model from the issue text.