Map thumbnail

Work from ROGUE on map thumbnails using printNG engine. This will generate a map thumbnail either using the wms reflector and the aggregate list of layers, or the user can select "Set Map Thumbnail" from the Map Detail page to use the current map view and background layers to construct a thumbnail.

NOTE: Did the minimal work to hide the options if printNG is not enabled in geoserver, but need to do further testing using a plain geoserver. Since our default build includes this module, I think we can safely pull this now and deal with 'plain vanilla' geoserver issues separately.