## 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"])
        _create_event(
            properties={"$current_url": "/", "test": "val"}, distinct_id="person_1", event="$pageview", team=self.team,
        )
        _create_event(
            properties={"$current_url": "/about", "test": "val"},
            distinct_id="person_1",
            event="$pageview",
            team=self.team,
        )
        _create_person(team=self.team, distinct_ids=["dontcount"])
        _create_event(
            properties={"$current_url": "/", "test": "val"}, distinct_id="dontcount", event="$pageview", team=self.team,
        )
        _create_event(
            properties={"$current_url": "/about", "test": "val"},
            distinct_id="dontcount",
            event="$pageview",
            team=self.team,
        )
        get_response = self.client.get(
            f"/api/projects/{self.team.id}/insights/path",
            data={"properties": json.dumps([{"key": "test", "value": "val"}]),},
        ).json()
        post_response = self.client.post(
            f"/api/projects/{self.team.id}/insights/path", {"properties": [{"key": "test", "value": "val"}],}
        ).json()
        self.assertEqual(len(get_response["result"]), 1)
        self.assertEqual(len(post_response["result"]), 1)
    def test_insight_funnels_basic_post(self):