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
It needs to be in that format b/c it's going into the SODA API, which is very dumb. I could do it as a global variable or something, is that what you're suggesting?