## 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
f"/api/projects/{self.team.id}/dashboards/%s/" % dashboard.pk,
            {"filters": {"date_from": "-24h"}},
            format="json",
        )
        self.assertEqual(patch_response.status_code, status.HTTP_200_OK)
        patch_response_json = patch_response.json()
        self.assertEqual(patch_response_json["items"][0]["result"], None)
        dashboard.refresh_from_db()
        self.assertEqual(dashboard.filters, {"date_from": "-24h"})
        
        self.assertEqual(insight_one_original_filter_hash, Insight.objects.get(pk=insight_one_id).filters_hash)
        self.assertEqual(insight_two_original_filter_hash, Insight.objects.get(pk=insight_two_id).filters_hash)
        
        tile_one = DashboardTile.objects.filter(insight__id=insight_one_id).first()
        if tile_one is None:
            breakpoint()
        self.assertEqual(
            patch_response_json["items"][0]["filters_hash"],
            tile_one.filters_hash
            if tile_one is not None
            else f"should have been able to load a single tile for {insight_one_id}",
        )
        tile_two = DashboardTile.objects.filter(insight__id=insight_two_id).first()
        self.assertEqual(
            patch_response_json["items"][1]["filters_hash"],
            tile_two.filters_hash
            if tile_two is not None
            else f"should have been able to load a single tile for {insight_two_id}",
        )