MDL-65888 forum: Do not send notifications to unconfirmed users
Under certain conditions, users marked as not confirmed yet may be
returned as forum notification recipients. Such users are considered as
inactive though and the delivery adhoc task cannot be run as these
users, throwing an exception.
The solution is to make sure that fetch_subscribed_users() does not
include unconfirmed users, similarly to what was done for suspended
users and nologin users.
The added unit test also checks for deleted users. But these should
never be returned as subscribed because we consider enrolled users only
and deleted users are filtered out implicitly.