## 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
self.assertEqual(response_3.status_code, status.HTTP_200_OK)
            self.assertDictContainsSubset(
                {
                    "created_at": "2021-08-23T12:00:00Z",
                    "created_by": self_user_basic_serialized,
                    "updated_at": "2021-10-21T12:00:00Z",
                    "last_modified_at": "2021-10-21T12:00:00Z",
                    "last_modified_by": self_user_basic_serialized,
                },
                response_3.json(),
            )
        with freeze_time("2021-12-23T12:00:00Z"):
            response_4 = self.client.patch(f"/api/projects/{self.team.id}/insights/{insight_id}", {"name": "XYZ"})
            self.assertEqual(response_4.status_code, status.HTTP_200_OK)
            self.assertDictContainsSubset(
                {
                    "created_at": "2021-08-23T12:00:00Z",
                    "created_by": self_user_basic_serialized,
                    "updated_at": "2021-12-23T12:00:00Z",
                    "last_modified_at": "2021-12-23T12:00:00Z",
                    "last_modified_by": self_user_basic_serialized,
                },
                response_4.json(),
            )
        
        self.client.force_login(alt_user)
        with freeze_time("2022-01-01T12:00:00Z"):
            response_5 = self.client.patch(
                f"/api/projects/{self.team.id}/insights/{insight_id}", {"description": "Lorem ipsum."}
            )