## 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
from rest_framework.settings import api_settings
from rest_framework_csv import renderers as csvrenderers
from rest_hooks.signals import raw_hook_event
from posthog.api.routing import StructuredViewSetMixin
from posthog.api.shared import UserBasicSerializer
from posthog.api.utils import get_target_entity
from posthog.auth import PersonalAPIKeyAuthentication, TemporaryTokenAuthentication
from posthog.celery import update_cache_item_task
from posthog.constants import INSIGHT_STICKINESS, TREND_FILTER_TYPE_ACTIONS, TREND_FILTER_TYPE_EVENTS, TRENDS_STICKINESS
from posthog.decorators import CacheType, cached_function
from posthog.models import (
    Action,
    ActionStep,
    CohortPeople,
    DashboardItem,
    Entity,
    Event,
    Filter,
    Person,
    RetentionFilter,
)
from posthog.models.event import EventManager
from posthog.models.filters.stickiness_filter import StickinessFilter
from posthog.models.team import Team
from posthog.permissions import ProjectMembershipNecessaryPermissions
from posthog.queries import base, retention, stickiness, trends
from posthog.tasks.calculate_action import calculate_action
from posthog.utils import generate_cache_key, get_safe_cache
from .person import PersonSerializer, paginated_result
class ActionStepSerializer(serializers.HyperlinkedModelSerializer):