## 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/action.py
team = self.team
        properties = request.GET.get("properties", "{}")
        data = {"properties": json.loads(properties)}
        start_entity_data = request.GET.get("start_entity", None)
        if start_entity_data:
            entity_data = json.loads(start_entity_data)
            data.update({"entites": [Entity({"id": entity_data["id"], "type": entity_data["type"]})]})
        data.update({"date_from": "-11d"})
        filter = RetentionFilter(data=data)
        result = retention.Retention().run(filter, team)
        return Response({"data": result})
    @action(methods=["GET"], detail=False)
    def funnel(self, request: request.Request, *args: Any, **kwargs: Any) -> Response: