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.
This is also [a publicly documented method](https://docs.djangoproject.com/en/dev/howto/custom-file-storage/#django.core.files.storage.get_available_name) and will require adjustment.