when the png resizing switch is on, don't rewrite the URL, on client side for 10% of image upgrades also put the resized png into a hidden img element
add a short sell by date for the png resizing switch
Merge branch 'master' into load-test-png-resizer
Merge pull request #7413 from guardian/load-test-png-resizer

send png resizer requests for all pngs on 10% of page loads