Add `/standalone/calibration` page

[eswan18]

Users need to be able to see how their forecasts, filtered over a certain time period, performed. How to measure that? I'm not entirely sure yet but I'll be using this as a starting point: https://fivethirtyeight.com/features/when-we-say-70-percent-it-really-means-70-percent/

[eswan18]

I haven't figured this out yet, since you'd need a lot of forecasts before this would be meaningful. I'll leave it open but probably won't get to it any time soon.