dovecot: Adapt mediator mailbox for new mediators

Link: https://lists.archlinux.org/archives/list/staff@lists.archlinux.org/message/3JNJQJ7O3LBIXST2EFBCAKM2HCHXJZUM/
Signed-off-by: Christian Heusel christian@heusel.eu

requested review from @klausenbusk

approved this merge request

Hmm, is there more setup needed? Somehow the mailbox is still not showing up for my account

$ sudo -u vmail doveadm mailbox list -u gromit
alerts
Junk
Trash
Drafts
Sent
INBOX

resolved all threads

mentioned in commit 971bc190

merged manually