Migrate importer_datapackage to contrib app

[ridoo]

Currently the datapackage import handler is just a python module. Looking forward to smoothen the GeoNode integration, we can think kind of configurable subtypes (this would need some refactoring, though, on some locations in GeoNode to handle each appropriately). However, having more control over settings, translations, etc. we should consider to migrate the module to a normal GeoNode contrib app.