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
The first step to cleaning it up would be to differentiate between the instance variables which are used in the view versus the ones which are used to perform the date calculations in the controller.