## 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
{"name": "insight new name", "description": "Internal system metrics.",}
            )
            short_id = insight["short_id"]
            frozen_time.tick(delta=timedelta(minutes=10))
            response = self.client.delete(f"/api/projects/{self.team.id}/insights/{insight_id}",)
            self.assertEqual(response.status_code, status.HTTP_204_NO_CONTENT)
            response = self.client.get(f"/api/projects/{self.team.id}/insights/{insight_id}",)
            self.assertEqual(response.status_code, status.HTTP_404_NOT_FOUND)
            self.assert_insight_activity(
                insight_id=None,
                expected=[
                    {
                        "user": {"first_name": "", "email": "user1@posthog.com"},
                        "activity": "deleted",
                        "created_at": "2012-01-14T03:31:34Z",
                        "scope": "Insight",
                        "item_id": str(insight_id),
                        "detail": {"changes": None, "merge": None, "name": "insight new name", "short_id": short_id},
                    },
                    {
                        "user": {"first_name": "", "email": "user1@posthog.com"},
                        "activity": "created",
                        "created_at": "2012-01-14T03:21:34Z",
                        "scope": "Insight",
                        "item_id": str(insight_id),
                        "detail": {"changes": None, "merge": None, "name": "insight new name", "short_id": short_id},
                    },
                ],
            )
    def test_cannot_set_filters_hash_via_api(self):