dovecot: Fix location of Inbox in dovecot 2.4

- Dovecot is upgraded from 2.3 to 2.4, users are unable to see the old mails
from before the upgrade. New mails can be received but old mails can't be
accessed. Old mails are still present in
/var/mail/{usernmame}/mail/mailboxes/... New mails are being stored in
/var/mail/{username}/u.*. Other mailboxes such as 'Sent' are not affected.

Tests:

- Mails received in the inbox before the upgrade to dovecot 2.4 are now visible.
Without the patch, pre-upgrade mails are not visible and newly received mails
are stored in /var/mail/{username} instead of /var/mail/{username}/mailboxes/...

Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/email/data/usr/share/freedombox/etc/dovecot/conf.d/2.4/15-freedombox-mail.conf

@@ -9,6 +9,7 @@
 # have enabled btrfs filesystem compression by default.
 mail_driver = sdbox
 mail_path = ~/mail
+mail_inbox_path =
 
 # We try to deliver all mail using a single UID 'mail' and a single GID 'mail'.
 # In Debian, UID of mail user is 8 and GID of mail user is 8 as set in