fix(mok): Add missing persistentence for sieve data

This patch fixes the absense of persistence for sieve rules, due to
being layed out in parallel to the `/srv/mail/mailbox` directory in the
dovecot container.

By moving the mountpoint to `/var/mail` the sieve rules should be
persisted along the way with the mailboxes.

BREAKING CHANGE: This patch adjusts the mount location for mail data,
which will require manual interaction.