## 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
if (self.action == "list" or self.action == "retrieve") and str_to_bool(
            self.request.query_params.get("basic", "0"),
        ):
            return InsightBasicSerializer
        return super().get_serializer_class()
    def get_queryset(self) -> QuerySet:
        queryset = super().get_queryset()
        if self.action == "list":
            queryset = queryset.filter(deleted=False)
            queryset = self._filter_request(self.request, queryset)
        order = self.request.GET.get("order", None)
        if order:
            queryset = queryset.order_by(order)
        else:
            queryset = queryset.order_by("order")
        return queryset
    def _filter_request(self, request: request.Request, queryset: QuerySet) -> QuerySet: