Graphite base url abstraction

I talked Thomas into setting up an https endpoint for graphite graphs. That lets us not have security warnings from the browser when those graphs are embedded on https pages. He had to set it up at a different hostname because the regular https://nanny.cul... is handled by pamacea, and he didn't see an easy way to just open up the rendering endpoint through that.

Switching that base URL in, I also pulled it out of the templates and into a setting that gets injected into the context.