[Workbench] Use download service instead of passing Keep data through Rails
- One possibility is to decode a Workbench cookie with configured rails-secret-token (suitable if nginx is mapping a namespace under workbench onto the downloader, or using a cookie domain1 to tell the browser to send the cookie to the downloader host as well)
Workbench needs a config knob for "base uri for downloads" (use built-in if not configured) if not relying on nginx mapping a namespace
Workbench needs to set a more permissive cookie domain or (if not relying on nginx mapping) use JS tricks for download links
1 Or a route that copies Authorization to an encrypted cookie?