## 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_dashboard.py
db_connection = connections[DEFAULT_DB_ALIAS]
        with CaptureQueriesContext(db_connection) as capture_query_context:
            response = self.client.get(f"/api/projects/{self.team.id}/dashboards/{dashboard_id}/")
            self.assertEqual(response.status_code, status.HTTP_200_OK)
            query_count = len(capture_query_context.captured_queries)
            return query_count, capture_query_context.captured_queries
    @snapshot_postgres_queries
    def test_adding_insights_is_not_nplus1_for_gets(self):