lib-storage, acl: Move hiding non-listable public/shared mailboxes to lib-storage

lib-storage, acl: Move hiding non-listable public/shared mailboxes to lib-storage

The subscriptions handling is now in lib-storage code, and ACL code doesn't
know about it. Change this so that if public/shared namespace has
subscriptions=yes (i.e. shared across all users), any nonexistent mailboxes'
subscriptions are hidden. This assumes that the user doesn't have enough
ACL rights to list the mailboxes, so they also don't want subscriptions for
them.