Send a notification mail to the users added to the "credit others" field

Added a @todo in here: https://git.drupalcode.org/project/contribution_records/-/blob/1.0.x/src...

The subject and body were taken from the D7 version and changed slightly. Ready for review.

This is now merged and it will be deployed shortly.

Now that this issue is closed, please review the contribution record.

As a contributor, attribute any organization helped you, or if you volunteered your own time.

Maintainers, please credit people who helped resolve this issue.

This is actually the first time we’ll be sending email from new.drupal.org, and this has reminded us that we hadn’t set up outgoing email yet. That infrastructure setup is in progress.