Cannot add layer to existing map

[giohappy]

The "Add to map" action doesn't work. This is a regression from the refactoring of the geonode plugin when showing existing maps. The original mechanism worked like this:

• the GeoNode view adds a layer passed as GET param to the viewer_obj
• however, the geonode plugin converter doesn't use that object any more. In case of an existing map it tries to not manipulate the MapStore blob config
• the layer passed as GET param is not inflated inside the final map config

The proposed solution is to obtain the layer from the GET param directly inside the GeoNode plugin, generate the MapStore config for the layer, and inflate it into the final map config.