## 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_insight.py
dashboard_id, _ = self._create_dashboard({"filters": {"date_from": "-14d",}})
        with freeze_time("2012-01-14T03:21:34.000Z"):
            _create_event(team=self.team, event="$pageview", distinct_id="1")
            _create_event(team=self.team, event="$pageview", distinct_id="2")
        with freeze_time("2012-01-15T04:01:34.000Z"):
            response = self.client.post(
                f"/api/projects/{self.team.id}/insights",
                data={"filters": {"events": [{"id": "$pageview"}]}, "dashboards": [dashboard_id]},
            ).json()
            self.assertEqual(response["last_refresh"], None)
            response = self.client.get(f"/api/projects/{self.team.id}/insights/{response['id']}/?refresh=true").json()
            self.assertEqual(spy_update_insight_cache.call_count, 1)
            self.assertEqual(response["result"][0]["data"], [0, 0, 0, 0, 0, 0, 2, 0])
            self.assertEqual(response["last_refresh"], "2012-01-15T04:01:34Z")
            self.assertEqual(response["last_modified_at"], "2012-01-15T04:01:34Z")
        with freeze_time("2012-01-15T05:01:34.000Z"):
            _create_event(team=self.team, event="$pageview", distinct_id="1")
            response = self.client.get(f"/api/projects/{self.team.id}/insights/{response['id']}/?refresh=true").json()
            self.assertEqual(spy_update_insight_cache.call_count, 2)
            self.assertEqual(response["result"][0]["data"], [0, 0, 0, 0, 0, 0, 2, 1])
            self.assertEqual(response["last_refresh"], "2012-01-15T05:01:34Z")