No need for explicit index on Resource properties

Similar to https://github.com/gae-init/gae-init/pull/72, removing explicit `indexed=True` for Resource properties in model; since they are candidate for indexing by default anyway (see https://developers.google.com/appengine/docs/python/ndb/properties#options and https://developers.google.com/appengine/docs/python/datastore/typesandpropertyclasses)