## 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
import json
from datetime import datetime, timedelta
from typing import Any, Dict, List, Optional, Tuple
from unittest.case import skip
from unittest.mock import patch
import pytz
from django.db import DEFAULT_DB_ALIAS, connections
from django.test.utils import CaptureQueriesContext
from django.utils import timezone
from freezegun import freeze_time
from rest_framework import status
from ee.api.test.base import LicensedTestMixin
from ee.clickhouse.util import ClickhouseTestMixin
from ee.models import DashboardPrivilege
from ee.models.explicit_team_membership import ExplicitTeamMembership
from posthog.models import (
    Cohort,
    Dashboard,
    DashboardTile,
    Filter,
    Insight,
    InsightViewed,
    Person,
    Team,
    User,
)
from posthog.models.organization import OrganizationMembership
from posthog.tasks.update_cache import update_insight_cache
from posthog.test.base import APIBaseTest, QueryMatchingTest, _create_event, _create_person
class TestInsight(ClickhouseTestMixin, LicensedTestMixin, APIBaseTest, QueryMatchingTest):