mail/rubygem-actionmailbox72: Add rubygem-actionmailbox72 7.2.1

Action Mailbox routes incoming emails to controller-like mailboxes for
processing in Rails. It ships with ingresses for Mailgun, Mandrill, Postmark,
and SendGrid. You can also handle inbound mails directly via the built-in Exim,
Postfix, and Qmail ingresses.

The inbound emails are turned into InboundEmail records using Active Record and
feature lifecycle tracking, storage of the original email on cloud storage via
Active Storage, and responsible data handling with on-by-default incineration.

These inbound emails are routed asynchronously using Active Job to one or
several dedicated mailboxes, which are capable of interacting directly with the
rest of your domain model.