## 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
"""
        Various regression tests for the serializer
        """
        
        response = self.client.get(
            f"/api/projects/{self.team.id}/insights/trend/?events={json.dumps([{'id': '$pageview'}])}&properties=%5B%5D&display=ActionsLineGraph"
        )
        self.assertEqual(patch_capture_exception.call_count, 0, patch_capture_exception.call_args_list)
        
        events = [{"id": "$pageview", "properties": [{"key": "something", "value": ["something"]}]}]
        response = self.client.get(
            f"/api/projects/{self.team.id}/insights/trend/?events={json.dumps(events)}&properties=%5B%5D&display=ActionsLineGraph"
        )
        self.assertEqual(patch_capture_exception.call_count, 0, patch_capture_exception.call_args_list)
        
        cohort_one_id = self._create_one_person_cohort([{"key": "prop", "value": 5, "type": "person"}])
        cohort_two_id = self._create_one_person_cohort([{"key": "prop", "value": 6, "type": "person"}])
        events = [
            {"id": "$pageview", "properties": [{"key": "something", "value": ["something"]}]},
            {"id": "$pageview"},
        ]
        response = self.client.post(
            f"/api/projects/{self.team.id}/insights/funnel/",
            {"events": events, "breakdown": [cohort_one_id, cohort_two_id], "breakdown_type": "cohort"},
        )
        self.assertEqual(response.status_code, 200)
        self.assertEqual(patch_capture_exception.call_count, 0, patch_capture_exception.call_args_list)
    def _create_one_person_cohort(self, properties: List[Dict[str, Any]]) -> int: