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.
Not sure there's a good reason to default to 100 -- the historic limit for `FileField` seems arbitrary. I'd suggest to default to `None` and let the caller override it as necessary.