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

The PNG resizer was getting unhealthy when we were using it in production.  We are not sure why as it works ok with light load.

This is to add some load to the resizer without affecting the user experience if it dies.  The browser will just GET a small image and store it in a hidden img element.

It's pretty hacky as the existing code's not really designed with this in mind, but I didn't want to rewrite it all just for a short term test.

I will turn on for short periods during the day and monitor the servers for response time etc.

@robertberry @phamann any comments?
I'd generally avoid commenting out code and adding comments like this - it's kind of what version control is for. Once you're done with this you ought to just be able to revert the whole branch (with a press of a button), so you ought not to need to add comments like this if you see what I mean ... 

(But don't bother removing them this time.)