## 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 posthog.models.filters.stickiness_filter import StickinessFilter
from posthog.permissions import ProjectMembershipNecessaryPermissions
from posthog.queries import paths, retention, stickiness, trends
from posthog.queries.sessions.sessions import Sessions
from posthog.utils import generate_cache_key, get_safe_cache, str_to_bool
class InsightBasicSerializer(serializers.ModelSerializer):
    """
    Simplified serializer to speed response times when loading large amounts of objects.
    """
    class Meta:
        model = DashboardItem
        fields = [
            "id",
            "short_id",
            "name",
            "filters",
            "dashboard",
            "color",
            "last_refresh",
            "refreshing",
            "saved",
        ]
        read_only_fields = ("short_id",)
    def create(self, validated_data: Dict, *args: Any, **kwargs: Any) -> Any:
        raise NotImplementedError()
    def to_representation(self, instance):
        representation = super().to_representation(instance)
        representation["filters"] = instance.dashboard_filters()
        return representation
class InsightSerializer(InsightBasicSerializer):