Track how long users are viewing images for

This is complete, but it would be better if the HEAD request
was actually aborted by Varnish when the viewDuration parameter is
present, or if the hit pointed to a script that does that.

Change-Id: I66cafd97427756411e967de1901324af2215e3ae
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/1001