## 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
import json
from typing import Any, Dict, List, Optional, Tuple
from dateutil import parser
from django.db import DEFAULT_DB_ALIAS, connection, connections
from django.test.utils import CaptureQueriesContext
from django.utils import timezone
from django.utils.timezone import now
from freezegun import freeze_time
from rest_framework import status
from posthog.models import Dashboard, DashboardTile, Filter, Insight, Team, User
from posthog.models.organization import Organization
from posthog.test.base import APIBaseTest, QueryMatchingTest, snapshot_postgres_queries
from posthog.utils import generate_cache_key
class TestDashboard(APIBaseTest, QueryMatchingTest):
    CLASS_DATA_LEVEL_SETUP = False
    @snapshot_postgres_queries
    def test_retrieve_dashboard_list(self):