## 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
from typing import Any, Dict, Type
from django.core.cache import cache
from django.db.models import QuerySet
from django.db.models.query_utils import Q
from django.utils.timezone import now
from django_filters.rest_framework import DjangoFilterBackend
from rest_framework import request, serializers, viewsets
from rest_framework.decorators import action
from rest_framework.permissions import IsAuthenticated
from rest_framework.response import Response
from posthog.api.routing import StructuredViewSetMixin
from posthog.api.shared import UserBasicSerializer
from posthog.api.utils import format_next_url
from posthog.celery import update_cache_item_task
from posthog.constants import (
    FROM_DASHBOARD,
    INSIGHT,
    INSIGHT_FUNNELS,
    INSIGHT_PATHS,
    INSIGHT_STICKINESS,
    TRENDS_STICKINESS,
)
from posthog.decorators import CacheType, cached_function
from posthog.models import DashboardItem, Event, Filter, Team
from posthog.models.filters import RetentionFilter
from posthog.models.filters.path_filter import PathFilter
from posthog.models.filters.sessions_filter import SessionsFilter