Fix download with WOPI integration

There is an issue with WOPI integration where Microsoft tested the "Download" functionnality, and they found a little bug : The end-user needs to refresh the browser to download the file.

An easy workaround to fix it is to add the property "allow-downloads" to the sandbox attribute of the IFRAME loading the Office editor in the WOPI Host Page.

Making this change to https://github.com/nuxeo/nuxeo/blob/master/modules/platform/nuxeo-wopi/src/main/resources/web/nuxeo.war/wopi.jsp#L68 with

// The sandbox attribute is needed to allow automatic redirection to the O365 sign-in page in the business user flow
 office_frame.setAttribute('sandbox', 'allow-scripts allow-same-origin allow-forms allow-popups allow-top-navigation allow-popups-to-escape-sandbox allow-downloads');

has fixed the problem.