Centralise notification management

It would be useful, on the odd occasion where something is necessarily a manual step (e.g. confirming an SNS topic recipient by clicking the link in an email) if we had an Ansible role we could include that would email a message to a set of recipients to notify them a manual step is necessary. We should certainly centralise that somehow. It could be a role or simply a variable in the existing _init role.

There is a mail module here:

• https://docs.ansible.com/ansible/latest/collections/community/general/mail_module.html