Add allow-downloads to iframe sandbox attributes

Google Chrome 83 began to disallow iframes from downloading unless the allow-downloads key is added to an iframe’s sandbox attributes:

As some Power-Ups would like to be able to trigger downloads upon user action, we have added the allow-downloads to the list of attributes.

With this change, the attributes currently included are:

allow-scripts allow-forms allow-popups allow-same-origin allow-popups-to-escape-sandbox allow-modals allow-presentation allow-storage-access-by-user-activation