## 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
@action(methods=["GET"], detail=False)
    def trend(self, request: request.Request, *args: Any, **kwargs: Any) -> Response:
        result = self.calculate_trends(request)
        filter = Filter(request=request)
        next = format_next_url(request, filter.offset, 20) if len(result["result"]) > 20 else None
        return Response({**result, "next": next})
    @cached_function()
    def calculate_trends(self, request: request.Request) -> Dict[str, Any]:
        team = self.team
        filter = Filter(request=request)
        if filter.insight == INSIGHT_STICKINESS or filter.shown_as == TRENDS_STICKINESS:
            earliest_timestamp_func = lambda team_id: Event.objects.earliest_timestamp(team_id)
            stickiness_filter = StickinessFilter(
                request=request, team=team, get_earliest_timestamp=earliest_timestamp_func
            )
            result = stickiness.Stickiness().run(stickiness_filter, team)
        else:
            result = trends.Trends().run(filter, team)
        self._refresh_dashboard(request=request)
        return {"result": result}
    
    
    #
    
    
    
    
    @action(methods=["GET"], detail=False)
    def session(self, request: request.Request, *args: Any, **kwargs: Any) -> Response: