User_model changes for #768

Makes the behavior and arguments for the find_all() method of the
User_model consistent with other models inheriting from BF_Model and
with BF_Model itself. User_model no longer defaults to hide deleted
users, so modules calling the User_model->find_all() method must be
updated accordingly. Additionally, calling find_all() and passing true
(expecting the old behavior of returning all users, including deleted
users) may result in an error, as it may change the return type.