## 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/models/insight.py
import secrets
import string
from typing import Optional
from django.contrib.postgres.fields.array import ArrayField
from django.db import models
from django.db.models.signals import pre_save
from django.dispatch import receiver
from django.utils import timezone
from django_deprecate_fields import deprecate_field
from posthog.models.dashboard import Dashboard
from posthog.models.filters.utils import get_filter
from posthog.utils import generate_cache_key
def generate_short_id():
    """Generate securely random 8 characters long alphanumeric ID."""
    return "".join(secrets.choice(string.ascii_letters + string.digits) for _ in range(8))
class Insight(models.Model):