Update heatmap on map resize

Heatmap layer didn't change its size after map initialization, even if the map itself was resized. It caused the heatmap to clip if map size was increased.

This fix also simplifies interaction with map events. Instead of viewreset and dragend, I only listen to moveend event, which fires whenever the map stops moving, zooming or resizing.

I used this example as a reference: https://github.com/Leaflet/Leaflet.heat/blob/gh-pages/src/HeatLayer.js