Fixed #9893 -- Filename could exceed max_length on a collision

Passing max_length argument into the Storage's save() and get_available_name() methods.
Rewriting get_available_name() so filenames do not exceed max_length when there is a collision.
Adding test for the case.
What about making the `max_length` kwarg default to `None` and fallback on `self.field.max_length` instead?