use leaflet.pm for drawing

*original title:* **snapping claim polygon points to other claims while drawing**

Claims submissions often don't exactly border existing claims although the submitter wanted them to (some offset by 1 block, some completely different points approximating the other claim's border).

This could be prevented by snapping to the points of other nearby polygons when drawing a new polygon.

There is a [Leaflet plugin](https://github.com/makinacorpus/Leaflet.Snap/) that does this. [Example usage](https://github.com/Leaflet/Leaflet.Editable/blob/99e421f/example/snapping.html#L79-L119)