Add an S3 storage plugin about indico-plugins

[ThiefMaster]

@mvidalgarcia suggested libcloud which looks VERY nice in terms of its Python API - when using that the plugin would be a generic cloud storage plugin instead of being S3-specific.

The only real drawback I see is that it seems to lack an equivalent of boto3's generate_presigned_url() method that generates a temporary public URL to download a specific file. While it might not be too hard to implement the signature generation, it would be problematic when used with something that isn't S3...

cloudstorage looks good as well, but unfornately it supports only Python 3. Besides that, it has hard dependencies against all the various cloud storage client libraries which isn't amazing since it pulls in a ton of new packages (especially for Azure). BTW, their docs indicate that libcloud does indeed lack support for generating signed URLs:

Generate temporary signed URLs for downloading and uploading files.