## 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_5.status_code, status.HTTP_200_OK)
            self.assertDictContainsSubset(
                {
                    "created_at": "2021-08-23T12:00:00Z",
                    "created_by": self_user_basic_serialized,
                    "updated_at": "2022-01-01T12:00:00Z",
                    "last_modified_at": "2022-01-01T12:00:00Z",
                    "last_modified_by": alt_user_basic_serialized,
                },
                response_5.json(),
            )
    def test_get_saved_insight_items(self):
        filter_dict = {
            "events": [{"id": "$pageview"}],
            "properties": [{"key": "$browser", "value": "Mac OS X"}],
        }
        Insight.objects.create(
            filters=Filter(data=filter_dict).to_dict(), saved=True, team=self.team, created_by=self.user,
        )
        
        Insight.objects.create(
            filters=Filter(data=filter_dict).to_dict(), team=self.team, created_by=self.user,
        )
        
        Insight.objects.create(filters=Filter(data=filter_dict).to_dict(), team=self.team)
        response = self.client.get(f"/api/projects/{self.team.id}/insights/", data={"saved": "true", "user": "true"})
        self.assertEqual(response.status_code, status.HTTP_200_OK)
        self.assertEqual(len(response.json()["results"]), 1)
        self.assertEqual(len(response.json()["results"][0]["short_id"]), 8)
    def test_get_favorited_insight_items(self):