## 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 = Dashboard.objects.create(team=self.team, name="dashboard")
        filter_dict = {
            "events": [{"id": "$pageview"}],
            "properties": [{"key": "$browser", "value": "Mac OS X"}],
        }
        with freeze_time("2020-01-04T13:00:01Z"):
            
            insight = Insight.objects.create(
                filters=Filter(data=filter_dict).to_dict(), team=self.team, last_refresh=now()
            )
            DashboardTile.objects.create(dashboard=dashboard, insight=insight)
        with freeze_time("2020-01-20T13:00:01Z"):
            response = self.client.get(f"/api/projects/{self.team.id}/dashboards/%s/" % dashboard.pk).json()
        self.assertEqual(response["items"][0]["result"], None)
        self.assertEqual(response["items"][0]["last_refresh"], None)
    def test_refresh_cache(self):
        dashboard = Dashboard.objects.create(team=self.team, name="dashboard")
        with freeze_time("2020-01-04T13:00:01Z"):
            
            item_default: Insight = Insight.objects.create(
                filters=Filter(
                    data={"events": [{"id": "$pageview"}], "properties": [{"key": "$browser", "value": "Mac OS X"}],}
                ).to_dict(),
                team=self.team,
                last_refresh=now(),
                order=0,
            )
            DashboardTile.objects.create(dashboard=dashboard, insight=item_default)