[feature] Link to device if openwisp-controller integration is enabled

[nemesifier]

If the integration with openwisp-controller is enabled and the node has a device relation, it would be great to add the link to the device detail page to the node objeects of the NetJSON NetworkGraph and then change the visualizer to display this link, probably a button would be best.

We could add the device ID somewhere specific in the node JSON (not in properties so it won't be visualized) and edit the code of the visualizer so that the device integration adds the JS logic to display the button.