## 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
_create_person(team=self.team, distinct_ids=["person_1"], properties={"foo": "bar"})
        whatever_cohort: Cohort = Cohort.objects.create(
            id=113,
            team=self.team,
            groups=[{"properties": [{"type": "person", "key": "foo", "value": "bar", "operator": "exact"}]}],
            last_calculation=timezone.now(),
        )
        whatever_cohort.calculate_people_ch(pending_version=0)
        with self.settings(USE_PRECALCULATED_CH_COHORT_PEOPLE=True):  
            response_user_property = self.client.get(
                f"/api/projects/{self.team.id}/insights/trend/?events={json.dumps([{'id': '$pageview'}])}&properties={json.dumps([{'type':'person','key':'foo','value':'bar'}])}"
            )
            response_precalculated_cohort = self.client.get(
                f"/api/projects/{self.team.id}/insights/trend/?events={json.dumps([{'id':'$pageview'}])}&properties={json.dumps([{'type':'cohort','key':'id','value':113}])}"
            )
        self.assertEqual(response_precalculated_cohort.status_code, 200)
        response_user_property_data = response_user_property.json()
        response_precalculated_cohort_data = response_precalculated_cohort.json()
        response_user_property_data.pop("last_refresh")
        response_precalculated_cohort_data.pop("last_refresh")
        self.assertEntityResponseEqual(
            response_user_property_data["result"], response_precalculated_cohort_data["result"]
        )
    def test_insight_trends_compare(self):