Allow inclusion of 6 months of data in heatmap

* Heatmap will be more accurate/useful with more data
* Fetch and use past 6 months of data for heatmap
* Reflect time period and count in heatmap legend
* Allow API query to start in June
You can look at the show page and then legend.js to see how they're used. I'd welcome a better way to pass in that information, but I can't just get rid of them.