## 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
the actual query data. This can speed things up if it's not needed.
        """
        filter_dict = {
            "events": [{"id": "$pageview"}],
        }
        Insight.objects.create(
            filters=Filter(data=filter_dict).to_dict(), team=self.team, short_id="12345678",
        )
        Insight.objects.create(
            filters=Filter(data=filter_dict).to_dict(), team=self.team, saved=True,
        )
        response = self.client.get(f"/api/projects/{self.team.id}/insights/?basic=true")
        self.assertEqual(response.status_code, status.HTTP_200_OK)
        self.assertEqual(len(response.json()["results"]), 2)
        self.assertEqual(
            list(response.json()["results"][0].keys()),
            [
                "id",
                "short_id",
                "name",
                "filters",
                "dashboards",
                "description",
                "last_refresh",
                "refreshing",
                "saved",
                "updated_at",
            ],
        )
    def test_listing_insights_does_not_nplus1(self):