## 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



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posthog/api/action.py
import json
from typing import Any, Dict, List, Optional, Union, cast
import posthoganalytics
from django.core.cache import cache
from django.db.models import Count, Exists, OuterRef, Prefetch, QuerySet
from django.db.models.signals import post_save
from django.dispatch import receiver
from django.utils.timezone import now
from rest_framework import authentication, request, serializers, viewsets
from rest_framework.decorators import action
from rest_framework.permissions import IsAuthenticated
from rest_framework.response import Response