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.
I think a better wording would be: "Backwards compatibility for storage backends without support for `max_length` will be removed in Django 2.0."