## 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
result = serializers.SerializerMethodField()
    created_by = UserBasicSerializer(read_only=True)
    class Meta:
        model = DashboardItem
        fields = [
            "id",
            "short_id",
            "name",
            "filters",
            "filters_hash",
            "order",
            "deleted",
            "dashboard",
            "layouts",
            "color",
            "last_refresh",
            "refreshing",
            "result",
            "created_at",
            "saved",
            "created_by",
        ]
        read_only_fields = (
            "created_by",
            "created_at",
            "short_id",
        )
    def create(self, validated_data: Dict, *args: Any, **kwargs: Any) -> DashboardItem: