## 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
filter_dict = {
                "events": [{"id": "$pageview"}],
                "properties": [{"key": "$browser", "value": "Mac OS X"}],
            }
            DashboardItem.objects.create(
                filters=Filter(data=filter_dict).to_dict(), team=self.team, created_by=self.user
            )
            
            DashboardItem.objects.create(filters=Filter(data=filter_dict).to_dict(), team=self.team)
            response = self.client.get("/api/insight/", data={"user": "true"}).json()
            self.assertEqual(len(response["results"]), 1)
        def test_get_saved_insight_items(self):
            filter_dict = {
                "events": [{"id": "$pageview"}],
                "properties": [{"key": "$browser", "value": "Mac OS X"}],
            }
            DashboardItem.objects.create(
                filters=Filter(data=filter_dict).to_dict(), saved=True, team=self.team, created_by=self.user,
            )
            
            DashboardItem.objects.create(
                filters=Filter(data=filter_dict).to_dict(), team=self.team, created_by=self.user,
            )
            
            DashboardItem.objects.create(filters=Filter(data=filter_dict).to_dict(), team=self.team)
            response = self.client.get("/api/insight/", data={"saved": "true", "user": "true"})
            self.assertEqual(response.status_code, status.HTTP_200_OK)
            self.assertEqual(len(response.json()["results"]), 1)
            self.assertEqual(len(response.json()["results"][0]["short_id"]), 8)
        def test_get_insight_by_short_id(self):