Support new requirejs path for declarativewidgets 0.5.0

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See jupyter/declarativewidgets#313.

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Should be able to just add the path. Can probably even leave the old one in there if desired for compat with older versions. But I think master/0.5 of dashboard_server is compatible with 0.5.0 of declarative widgets and up only already: dashboard.js invokes the widget system init function using the new API, not the one from 0.4.x.

#194 covers moving to ipywidgets 5.0.

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We still need to get this one done for 0.5. I moved the other issues to 0.6 to separate them out.

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This seems to be the right stuff, though the map is a bit superfluous.

        // setup module paths used by plugins
        window.require.config({
            map: {
                '*': {
                    'nbextensions/widgets/widgets/js/widget': 'jupyter-js-widgets',
                    'jupyter-decl-widgets': 'urth_widgets/js/widgets'
                }
            }
        });

        window.define('jupyter-js-widgets', function() {
            return Widgets;
        });

@lbustelo: After adding this, hit a new incompatibility introduced by jupyter/declarativewidgets#322

if(!model.comm.kernel.comm_manager.comms[model.comm.comm_id]){
    this._handleCommDisconnect();
    return;
}

In dashboard server, model.comm does not have a kernel so it throws an exception and nothing loads. That model and comm are setup by the widgets services-shim.js which is what bridges jupyter-js-services into the notebook-like structure.

@jhpedemonte not sure if we can shim.

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Removing this fixes everything:

if(!model.comm.kernel.comm_manager.comms[model.comm.comm_id]){
  this._handleCommDisconnect();
  return;
}

@lbustelo since this is a hack to protect against a timing bug in ipywidgets, would it be possible to guard it with if(model.comm.kernel && (!model.comm.kernel.comm_manager.comms[model.comm.comm_id])).