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.
See https://docs.djangoproject.com/en/dev/internals/contributing/writing-code/submitting-patches/#deprecating-a-feature for testing a deprecated feature.