Override system default /etc/aliases DB with our template. Refs #2499
Fix admin's mailbox migration. Refs #2492

The admin user is now in accounts DB.
admin account is now in LDAP. Refs #2492

It does not need special treatment anymore.
Import mail from root mailbox to admin. Refs #2492
Deliver root's mail to admin. Refs #2492 #2499
List accounts to override /etc/aliases settings. Refs #2499

Added 40accounts fragment. If an account has no mail forward address
defined, set an identity forward value to override /etc/aliases.
nethserver-mail-update-admin action: signal user-modify admin event. Refs #2492

This action comes into play when mail-server is installed after
nethserver-directory: the admin account exists and needs to be
updated. It checks the LDAP accountStatus attribute.
Set local mail address if none is defined. Refs #2492

The "mail" LDAP attribute is used by SOGo to set up user account: the
blank value was not accepted.