Minimal zoom implementation

Just a link to the full-size file for now.

Since the link must be to a PNG/JPEG/GIF (so possibly a thumbnail),
and we want to cap the size, we might need to get the URL from the
API, but we need to open the new window right away to avoid popup
blockers, making this patch quite complicated.

Change-Id: I9ce9d2a2d27b75470eae2806d9f9ce2f95f4dac2
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/588