## 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")
        with freeze_time("2020-01-04T13:00:01Z"):
            
            item_default: DashboardItem = DashboardItem.objects.create(
                dashboard=dashboard,
                filters=Filter(
                    data={"events": [{"id": "$pageview"}], "properties": [{"key": "$browser", "value": "Mac OS X"}],}
                ).to_dict(),
                team=self.team,
                last_refresh=now(),
            )
            item_sessions: DashboardItem = DashboardItem.objects.create(
                dashboard=dashboard,
                filters=Filter(
                    data={
                        "display": "ActionsLineGraph",
                        "events": [{"id": "$pageview", "type": "events", "order": 0, "properties": []}],
                        "filters": [],
                        "insight": "SESSIONS",
                        "interval": "day",
                        "pagination": {},
                        "session": "avg",
                    }
                ).to_dict(),
                team=self.team,
                last_refresh=now(),
            )
        with freeze_time("2020-01-20T13:00:01Z"):
            response = self.client.get("/api/dashboard/%s?refresh=true" % dashboard.pk)