## 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/test/test_insight.py
"""
            The `skip_results` query parameter can be passed so that only a list of objects is returned, without
            the actual query data. This can speed things up if it's not needed.
            """
            filter_dict = {
                "events": [{"id": "$pageview"}],
            }
            DashboardItem.objects.create(
                filters=Filter(data=filter_dict).to_dict(), team=self.team, short_id="12345678",
            )
            DashboardItem.objects.create(
                filters=Filter(data=filter_dict).to_dict(), team=self.team, saved=True,
            )
            response = self.client.get("/api/insight/?basic=true")
            self.assertEqual(response.status_code, status.HTTP_200_OK)
            self.assertEqual(len(response.json()["results"]), 2)
            self.assertEqual(
                list(response.json()["results"][0].keys()),
                ["id", "short_id", "name", "filters", "dashboard", "color", "last_refresh", "refreshing", "saved"],
            )
        def test_create_insight_items(self):