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.
```py
warnings.warn(
    'foo'
    '...',
    RemovedInDjango20Warning, stacklevel=2
)
```