Services as core module

Moved services out of contrib.

Layer now has foreign key to service, typename no longer unique

Changed how layers are looked up (typename only for local layers, service + typename for remote): https://github.com/mbertrand/cga-worldmap/blob/services_gn/geonode/layers/views.py#L79

Make use of GXP lazy-loading to avoid GetCapabilities requests: 
https://github.com/mbertrand/cga-worldmap/blob/services_gn/geonode/layers/views.py#L170
https://github.com/mbertrand/cga-worldmap/blob/services_gn/geonode/maps/views.py#L387

Note: Latest django-geoexplorer (4.0.4) doesn't seem to have a GXP fix for an issue with ArcGIS REST layers that was merged a couple weeks ago: https://github.com/GeoNode/gxp/pull/1