## 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
filter_dict = {
            "events": [{"id": "$pageview"}],
        }
        insight_1 = Insight.objects.create(
            filters=Filter(data=filter_dict).to_dict(), team=self.team, short_id="12345678",
        )
        insight_2 = Insight.objects.create(
            filters=Filter(data=filter_dict).to_dict(), team=self.team, short_id="98765432",
        )
        self.client.post(f"/api/projects/{self.team.id}/insights/{insight_1.id}/viewed")
        self.client.post(f"/api/projects/{self.team.id}/insights/{insight_2.id}/viewed")
        response = self.client.get(
            f"/api/projects/{self.team.id}/insights/?my_last_viewed=true&order=-my_last_viewed_at"
        )
        response_data = response.json()
        
        self.assertEqual(response.status_code, status.HTTP_200_OK)