Allow to select any user as receiver of a notification workflow

[andergmartins]

Right now we can only select "authors" in the Users field. I think that limitation was added for limiting the number of users displayed on the page and avoiding performance issues.

For now, there is a snippet code that can bypass that limitation:

add_filter('publishpress_notif_users_select_form_get_users_args', 'allow_all_users_on_notifications');
function allow_all_users_on_notifications($args)
{
    unset($args['who']);

    return $args;
}

We should consider converting the user field into an ajax based field, loading data dynamically to avoid the performance issue for big sites.