webhook during release process

At the end of `publish_release.py` contact this URL:

`curl https://www.boost.io/admin/versions/version/new_version/`

That will trigger an import on boost.io to refresh the releases page of the website.

publish_release.py uploads files to S3. There can be a minute delay while the CDN origin servers download the content from S3. Therefore, the script should wait and loop, checking if the files are available on archives.boost.io, before contacting the webhook.