## 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/dashboard.py
request = self.context["request"]
        validated_data["created_by"] = request.user
        team = Team.objects.get(id=self.context["team_id"])
        use_template: str = validated_data.pop("use_template", None)
        creation_mode = "template" if use_template else "default"
        dashboard = Dashboard.objects.create(team=team, creation_mode=creation_mode, **validated_data)
        if use_template:
            try:
                create_dashboard_from_template(use_template, dashboard)
            except AttributeError:
                raise serializers.ValidationError({"use_template": "Invalid value provided."})
        elif request.data.get("items"):
            for item in request.data["items"]:
                DashboardItem.objects.create(
                    **{key: value for key, value in item.items() if key not in ("id", "deleted", "dashboard", "team")},
                    dashboard=dashboard,
                    team=team,
                )
        posthoganalytics.capture(
            request.user.distinct_id,
            "dashboard created",
            {**dashboard.get_analytics_metadata(), "from_template": bool(use_template), "template_key": use_template},
        )
        return dashboard
    def update(self, instance: Dashboard, validated_data: Dict, *args: Any, **kwargs: Any,) -> Dashboard: