Respect max_length when AWS_S3_FILE_OVERWRITE=True

[a1Gupta]

get_available_name method of S3Boto3Storage doesn't respect max_length.

The Django Storage's get_available_name method truncates the file name if it's length is more than max_length.

When AWS_S3_FILE_OVERWRITE=True and len(file_name) > max_length, this results in Database error - DataError: (1406, "Data too long for column 'file_field' at row 1").

    def get_available_name(self, name, max_length=None):
        """Overwrite existing file with the same name."""
        if self.file_overwrite:
            name = self._clean_name(name)
           >>  ** CHANGE REQUIRED HERE: max_length VALIDATION  **
            return name
        return super(S3Boto3Storage, self).get_available_name(name, max_length)

Validation for max_length should be added to the overridden method, when file_overwrite=True.

[jschneier]

Have opened provisional PR at #554