## 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/action.py
id = serializers.CharField(read_only=False, required=False)
    class Meta:
        model = ActionStep
        fields = [
            "id",
            "event",
            "tag_name",
            "text",
            "href",
            "selector",
            "url",
            "name",
            "url_matching",
            "properties",
        ]
        extra_kwargs = {
            "event": {"trim_whitespace": False},
            "tag_name": {"trim_whitespace": False},
            "text": {"trim_whitespace": False},
            "href": {"trim_whitespace": False},
            "name": {"trim_whitespace": False},
        }
class ActionSerializer(serializers.HyperlinkedModelSerializer):