Remove direct call to model in managers

This is simple change that is very important. Models shouldn't be call directly because this blocks possibility of inheriting them by other models without creating custom manager that solves this problem.

Models should never be call in this way in managers. The best solution is to use or `self.get_queryset()` method or `self.model` object representing current model. Also we can use that we have `Manager` object, so we can call any method.