Add ?download parameter to images

This parameter triggers Content-Disposition: Attachment which
makes the browser download the image instead of displaying it.

This is needed by Media Viewer to allow users to click a button in
order to download an image.

Change-Id: Ia8e15fd3f731b0b85e6985f5adaafeb6d8c32990
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/385