Added chart-ready callback for image generation

I've tested the basics of this callback locally, but I won't be able to properly test this until the code is live somewhere publicly accessible. It's harmless code, though — basically, if window.callPhantom isn't defined (which it won't be on any normal browser), it's completely ignored. But it's injected in slimerjs/phantomjs, so it'll run when it's loaded via slimer. 

Then it should fire that callback when the chart is rendered. Right now the server has a 6-second timeout, so if the chart doesn't fire the callback before then (it should), it'll just take the screenshot of what's rendered. I think I should be able to test this on staging by hard-coding my user/pass into the url, so... yeah, let me know if that works for you!