## 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
response = self.client.post("/api/dashboard/", {"name": "another", "use_template": "DEFAULT_APP"})
        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
        self.assertGreater(DashboardItem.objects.count(), 1)
        self.assertEqual(response.json()["creation_mode"], "template")
    def test_return_cached_results_dashboard_has_filters(self):
        
        dashboard = Dashboard.objects.create(team=self.team, name="dashboard")
        filter_dict = {
            "events": [{"id": "$pageview"}],
            "properties": [{"key": "$browser", "value": "Mac OS X"}],
            "date_from": "-7d",
        }
        filter = Filter(data=filter_dict)