Enable pubsub feature usage on nuxeo open social container

Enable pubsub feature usage on nuxeo open social container.

To enable pubsub, the gadgets.pubsubrouter should be initialized.
To initialize the pubsubrouter, a method to get gadget spec url is required.
All this updates affect the class org.nuxeo.opensocial.container.client.presenter.AppPresenter in the project addons>nuxeo-opensocial-container.

NB: To enable pubsub on your container, include the container side of the pubsub feature and initialize each gadget using gadgets.rpc.setupReceiver(yourGadgetId).