## 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/test/test_dashboard.py
dashboard_id, _ = self._create_dashboard({"name": "asdasd", "pinned": True})
        insight_id, _ = self._create_insight(
            {"filters": {"hello": "test"}, "dashboards": [dashboard_id], "name": "another"},
        )
        
        response = self.client.patch(
            f"/api/projects/{self.team.id}/dashboards/{dashboard_id}",
            {
                "tile_layouts": [
                    {
                        "id": insight_id,
                        "layouts": {
                            "lg": {"x": "0", "y": "0", "w": "6", "h": "5"},
                            "sm": {"w": "7", "h": "5", "x": "0", "y": "0", "moved": "False", "static": "False",},
                            "xs": {"x": "0", "y": "0", "w": "6", "h": "5"},
                            "xxs": {"x": "0", "y": "0", "w": "2", "h": "5"},
                        },
                    }
                ]
            },
            format="json",
        )
        self.assertEqual(response.status_code, status.HTTP_200_OK)
        dashboard_json = self.client.get(
            f"/api/projects/{self.team.id}/dashboards/{dashboard_id}/", {"refresh": False}
        ).json()
        first_tile_layouts = dashboard_json["items"][0]["layouts"]
        self.assertTrue("lg" in first_tile_layouts)
    def test_dashboard_from_template(self):