## 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
Person.objects.create(team=self.team, properties=properties)
        cohort_one_id = self.client.post(
            f"/api/projects/{self.team.id}/cohorts", data={"name": "whatever", "groups": [{"properties": properties}]},
        ).json()["id"]
        return cohort_one_id
    @freeze_time("2022-03-22T00:00:00.000Z")
    def test_create_insight_viewed(self):