'Overwrite of the original method to always follow user_id field,
even when not track_visibility so that a user will follow it\'s employee'

@api.model
def _message_get_auto_subscribe_fields(self, updated_fields, auto_follow_fields=None):

    if (auto_follow_fields is None):
        auto_follow_fields = ['user_id']
    user_field_lst = []
    for (name, field) in self._fields.items():
        if ((name in auto_follow_fields) and (name in updated_fields) and (field.comodel_name == 'res.users')):
            user_field_lst.append(name)
    return user_field_lst
