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/templates/dovecot.conf.j2

@@ -101,7 +101,7 @@ userdb {
 }
 userdb {
 	driver = passwd-file
-	args = username_format=%{original_user} /etc/dovecot/shared-mailboxes-acl
+	args = username_format=%{original_username} /etc/dovecot/shared-mailboxes-acl
 	# https://doc.dovecot.org/configuration_manual/config_file/config_variables/
 	override_fields = uid=vmail gid=vmail mail=mdbox:~/.mdbox:INDEXPVT=~/indexes/%Ln
 }