## 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
event_factory(team=self.team, event="user signed up", distinct_id="1")
                response = self.client.get(
                    "/api/insight/funnel/?events={}".format(
                        json.dumps([{"id": "user signed up", "type": "events", "order": 0},])
                    )
                ).json()
                self.assertEqual(response["result"]["loading"], True)
            
            def test_insight_retention_basic(self):
                person_factory(team=self.team, distinct_ids=["person1"], properties={"email": "person1@test.com"})
                event_factory(
                    team=self.team,
                    event="$pageview",
                    distinct_id="person1",
                    timestamp=timezone.now() - timedelta(days=11),
                )
                event_factory(
                    team=self.team,
                    event="$pageview",
                    distinct_id="person1",
                    timestamp=timezone.now() - timedelta(days=10),
                )
                response = self.client.get("/api/insight/retention/",).json()
                self.assertEqual(len(response["result"]), 11)
    return TestInsight
class TestInsight(insight_test_factory(Event.objects.create, Person.objects.create)):  
    pass