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
What does this date represent? Can you use a constant for it instead of a "magic number". Does it need to be a string, or can we actually use a DateTime