## Is your feature request related to a problem?

A user mentioned needing a feature to clone dashboards as they create the same dashboard multiple times, each for a different end-customer. Given that we are already planning on doing default / custom dashboards (#1659) we can add this functionality to facilitate dashboard creation.

## Describe the solution you'd like

Simple functionality to duplicate a dashboard.
- Add "Duplicate" button to dashboard list and clone dashboard (e.g. "Copy - AARRR").
![screenshot](https://user-images.githubusercontent.com/5864173/93866745-fce46100-fcbf-11ea-8c96-b1acfb1981d5.png)


## Describe alternatives you've considered
Re-create the dashboard manually.


## Additional context
No additional context.


#### *Thank you* for your feature request – we love each and every one!

posthog/api/test/test_dashboard.py
dashboard = Dashboard.objects.create(team=self.team, name="dashboard")
        filter_dict = {
            "events": [{"id": "$pageview"}],
            "properties": [{"key": "$browser", "value": "Mac OS X"}],
        }
        with freeze_time("2020-01-04T13:00:01Z"):
            
            DashboardItem.objects.create(
                dashboard=dashboard, filters=Filter(data=filter_dict).to_dict(), team=self.team, last_refresh=now()
            )
        with freeze_time("2020-01-20T13:00:01Z"):
            response = self.client.get("/api/dashboard/%s/" % dashboard.pk).json()
        self.assertEqual(response["items"][0]["result"], None)
        self.assertEqual(response["items"][0]["last_refresh"], None)
    def test_refresh_cache(self):