## 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
CLASS_DATA_LEVEL_SETUP = False
    def test_retrieve_dashboard(self):
        dashboard = Dashboard.objects.create(
            team=self.team, name="private dashboard", created_by=self.user, tags=["deprecated"]
        )
        response = self.client.get(f"/api/dashboard/{dashboard.id}")
        self.assertEqual(response.status_code, status.HTTP_200_OK)
        response_data = response.json()
        self.assertEqual(response_data["name"], "private dashboard")
        self.assertEqual(response_data["description"], "")
        self.assertEqual(response_data["tags"], ["deprecated"])
        self.assertEqual(response_data["created_by"]["distinct_id"], self.user.distinct_id)
        self.assertEqual(response_data["created_by"]["first_name"], self.user.first_name)
        self.assertEqual(response_data["creation_mode"], "default")
    def test_create_basic_dashboard(self):
        response = self.client.post("/api/dashboard/", {"name": "My new dashboard"})
        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
        response_data = response.json()
        self.assertEqual(response_data["name"], "My new dashboard")
        self.assertEqual(response_data["description"], "")
        self.assertEqual(response_data["tags"], [])
        self.assertEqual(response_data["creation_mode"], "default")
        instance = Dashboard.objects.get(id=response_data["id"])
        self.assertEqual(instance.name, "My new dashboard")
    def test_update_dashboard(self):