Make it clearer how temp files are created during decryption

[sssoleileraaa]

securedrop-client/securedrop_client/utils.py

Lines 92 to 93 in 94efe51

	TODO: Figure out a clearer way to safely copy to a temporary file that gets
	deleted right away. We may need a safe_decrypt function in the future.

Address this TODO and match how safe_move and safe_copyfileobj use safe_mkdir to make sure any directories that are created during move or copy are created with the correct perms and within the expected base_path.

We might also want to entirely depend on the umask setting for this to simplify code. If we rely on umask then we can remove the code that creates each subdir individually in safe_mkdir as well. Or we can leave it for defence in depth.