dovecot: Add shared mailbox for the mediation team

Please see [1] and [2] for a better understanding of how this works.

[1] https://doc.dovecot.org/configuration_manual/authentication/password_databases_passdb/ [2] https://doc.dovecot.org/configuration_manual/authentication/user_databases_userdb/

Fix #464 (closed)

roles/dovecot/files/shared-mailboxes

+mediation-team:::::::