Add cases over previous 7 days / 100000 population

[wbunting]

Would be super helpful to see which prefectures fall above / below the new guidelines of 0.5 infections per 100000 over the previous 7 days.

As far as I can tell all the data is there except for prefecture populations?

cc: @reustle

[liquidx]

I didn't quite know what that number was, but I seemed to find a reference to it in this news article:
https://www.nippon.com/en/news/ntv20200514002/panel-of-experts-offer-criteria-for-lifting-state-of-emergency.html

There should be some data around for prefecture populations, and so it's could be just be a threshold we show per-prefecture on whether they're close to that number of not.

[reustle]

Looks like it's all prefectures now?