Even better map

Upgrade to bettermap: you can now select regions on the map by drawing a polygon. The regions are linked to filters. You can remove either the filter or the region on the map, both will work.
To accomodate this feature I have also: 
* upgraded leaflet, 
* upgraded the marker cluster plugin, 
* added the leaflet.draw plugin. 
* made the OSM url configurable

Finally, I have added some code the the filterServer and to the filtering panel to accomodate geo_polygon filter types.