Provide the sha256 and sha512 of the releases
    
    With this commit when a new release is uploading we create or update a
    `checksums` file in the release folder containing both the sha256 and
    the sha512 of the file(s) uploaded.
    
    Fixes https://pagure.io/pagure/issue/2622
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>