## 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
class Meta:
        constraints = [
            models.UniqueConstraint(fields=["team", "user", "insight"], name="posthog_unique_insightviewed"),
        ]
        indexes = [
            models.Index(fields=["team_id", "user_id", "-last_viewed_at"]),
        ]
    team: models.ForeignKey = models.ForeignKey("Team", on_delete=models.CASCADE)
    user: models.ForeignKey = models.ForeignKey("User", on_delete=models.CASCADE)
    insight: models.ForeignKey = models.ForeignKey(Insight, on_delete=models.CASCADE)
    last_viewed_at: models.DateTimeField = models.DateTimeField()
@receiver(pre_save, sender=Insight)
def insight_saving(sender, instance: Insight, **kwargs):
    update_fields = kwargs.get("update_fields")
    if update_fields in [frozenset({"filters_hash"}), frozenset({"last_refresh"}), frozenset({"filters"})]:
        
        return
    
    if instance.filters and instance.filters != {}:
        instance.filters_hash = generate_insight_cache_key(instance, None)
def generate_insight_cache_key(insight: Insight, dashboard: Optional[Dashboard]) -> str:
    dashboard_insight_filter = get_filter(data=insight.dashboard_filters(dashboard=dashboard), team=insight.team)
    candidate_filters_hash = generate_cache_key("{}_{}".format(dashboard_insight_filter.toJSON(), insight.team_id))
    return candidate_filters_hash