## 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
legacy_team_compatibility = True
class DashboardItemSerializer(serializers.ModelSerializer):
    result = serializers.SerializerMethodField()
    last_refresh = serializers.SerializerMethodField()
    _get_result: Optional[Dict[str, Any]] = None
    class Meta:
        model = DashboardItem
        fields = [
            "id",
            "short_id",
            "name",
            "description",
            "filters",
            "filters_hash",
            "order",
            "deleted",
            "dashboard",
            "dive_dashboard",
            "layouts",
            "color",
            "last_refresh",
            "refreshing",
            "result",
            "is_sample",  
            "saved",
            "created_at",
            "created_by",
            "updated_at",
        ]
    def create(self, validated_data: Dict, *args: Any, **kwargs: Any) -> DashboardItem: