Each user should have a home that isn't their root maildir

[nextgens]

Each user should have a home that isn't their root maildir.

This would solve the problems given by .dovecot.lda-dupes and sieve scripts

[Diman0]

https://doc.dovecot.org/configuration_manual/mail_location/
Dovecot documentation with more information on how to configure this.

Ideally we split per domain.
e.g. /mailu/mail///

[nextgens]

We should probably deal with #1220 on the same PR too