## 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({"filters": {"date_from": "-14d"}})
        insight_id, _ = self._create_insight(
            {"filters": {"hello": "test", "date_from": "-7d"}, "dashboards": [dashboard_id], "name": "some_item"}
        )
        response = self.client.get(f"/api/projects/{self.team.id}/dashboards/{dashboard_id}/").json()
        self.assertEqual(len(response["items"]), 1)
        self.assertEqual(response["items"][0]["name"], "some_item")
        self.assertEqual(response["items"][0]["filters"]["date_from"], "-14d")
        item_response = self.client.get(f"/api/projects/{self.team.id}/insights/").json()
        self.assertEqual(item_response["results"][0]["name"], "some_item")
        
        self.client.patch(
            f"/api/projects/{self.team.id}/insights/{item_response['results'][0]['id']}/", {"deleted": "true"}
        )
        items_response = self.client.get(f"/api/projects/{self.team.id}/insights/").json()
        self.assertEqual(len(items_response["results"]), 0)
    def test_dashboard_filter_is_applied_even_if_insight_is_created_before_dashboard(self):