## Is your feature request related to a problem?

When setting up dashboards I'd like to filter the stats to a certain page/subdomain. 

## Describe the solution you'd like

Allow adding a extra filters for all graphs on a dashboard (e.g. filter by $current_url)

## Describe alternatives you've considered

Updating every graph in dashboard one-by-one and then reverting them back.

## Additional context

Related to https://github.com/PostHog/posthog/issues/780

Datadog has a decent solution for this: https://docs.datadoghq.com/dashboards/template_variables/, which I think we can simplify and use ourselves.

Came up during #2560 

#### *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)
        use_dashboard: int = validated_data.pop("use_dashboard", None)
        validated_data = self._update_creation_mode(validated_data, use_template, use_dashboard)
        tags = validated_data.pop("tags", None)  
        dashboard = Dashboard.objects.create(team=team, **validated_data)
        if use_template:
            try:
                create_dashboard_from_template(use_template, dashboard)
            except AttributeError:
                raise serializers.ValidationError({"use_template": "Invalid value provided."})
        elif use_dashboard:
            try:
                from posthog.api.insight import InsightSerializer
                existing_dashboard = Dashboard.objects.get(id=use_dashboard, team=team)
                existing_tiles = DashboardTile.objects.filter(dashboard=existing_dashboard).select_related("insight")
                for existing_tile in existing_tiles:
                    new_data = {
                        **InsightSerializer(existing_tile.insight, context=self.context,).data,
                        "id": None,  
                        "last_refresh": now(),
                    }
                    new_data.pop("dashboards", None)
                    new_tags = new_data.pop("tags", None)
                    insight_serializer = InsightSerializer(data=new_data, context=self.context,)
                    insight_serializer.is_valid()
                    insight_serializer.save()