## 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
items = serializers.SerializerMethodField()
    created_by = UserBasicSerializer(read_only=True)
    use_template = serializers.CharField(write_only=True, allow_blank=True, required=False)
    class Meta:
        model = Dashboard
        fields = [
            "id",
            "name",
            "description",
            "pinned",
            "items",
            "created_at",
            "created_by",
            "is_shared",
            "share_token",
            "deleted",
            "creation_mode",
            "use_template",
            "filters",
            "tags",
        ]
        read_only_fields = ("creation_mode",)
    def create(self, validated_data: Dict, *args: Any, **kwargs: Any) -> Dashboard: