## Bug description

We've had a user create a funnel with Korean characters and 17 steps, which causes this issue:
![image](https://user-images.githubusercontent.com/1727427/120301818-8c212a00-c2cd-11eb-8b00-7ba242e4a9ab.png)

## Expected behavior

We should send requests to api/insight/funnel using POST instead of GET.

## How to reproduce

1.
2.
3.

## Environment

- [ ] PostHog Cloud
- [ ] self-hosted PostHog, version/commit: _please provide_

## Additional context



#### *Thank you* for your bug report – we love squashing them!

posthog/api/insight.py
team = self.team
        refresh = request.GET.get("refresh", None)
        filter = Filter(request=request, data={"insight": INSIGHT_FUNNELS})
        cache_key = generate_cache_key("{}_{}".format(filter.toJSON(), team.pk))
        result = {"loading": True}
        if refresh:
            cache.delete(cache_key)
        else:
            cached_result = get_safe_cache(cache_key)
            if cached_result:
                task_id = cached_result.get("task_id", None)
                if not task_id:
                    return cached_result["result"]
                else:
                    return {"result": result}
        payload = {"filter": filter.toJSON(), "team_id": team.pk}
        task = update_cache_item_task.delay(cache_key, CacheType.FUNNEL, payload)
        if not task.ready():
            task_id = task.id
            cache.set(cache_key, {"task_id": task_id}, 180)  
        self._refresh_dashboard(request=request)
        return {"result": result}
    
    
    
    
    
    
    @action(methods=["GET"], detail=False)
    def retention(self, request: request.Request, *args: Any, **kwargs: Any) -> Response: