[admin] Integrate topology visualizer in admin

[nemesifier]

Integrate topology visualizer in the admin in some way. A good way would be a button that displays an overlay like we do in openwisp-controller for the preview feature, but you may take a shortcut and just link the visualizer in the frontend: it depends from how much you want to learn:

1. just adding a link to the visualizer in the frontend is a shortcut that will be fast to implement and won't teach you much
2. visualizer in overlay will take a few days longer but will teach you stuff you probably don't know about django's internals

You may also consider to do both in a very agile way, first you quickly implement 1 and then you go ahead and implement 2, this approach can help you learn more about incremental development and fast iteration!

The choice is up to you @rohithasrk.

[rohithasrk]

Implemented in openwisp/django-netjsongraph#36

[nemesifier]

@rohithasrk we'll just need to perform some manual tests once openwisp/django-netjsongraph#36 is ready