## 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
queryset = queryset.annotate(count=Count(TREND_FILTER_TYPE_EVENTS))
    queryset = queryset.prefetch_related(Prefetch("steps", queryset=ActionStep.objects.order_by("id")))
    return queryset.filter(team_id=team_id).order_by("-id")
class ActionViewSet(StructuredViewSetMixin, viewsets.ModelViewSet):
    renderer_classes = tuple(api_settings.DEFAULT_RENDERER_CLASSES) + (csvrenderers.PaginatedCSVRenderer,)
    queryset = Action.objects.all()
    serializer_class = ActionSerializer
    authentication_classes = [
        TemporaryTokenAuthentication,
        PersonalAPIKeyAuthentication,
        authentication.SessionAuthentication,
        authentication.BasicAuthentication,
    ]
    permission_classes = [IsAuthenticated, ProjectMembershipNecessaryPermissions]
    def get_queryset(self):
        queryset = super().get_queryset()
        if self.action == "list":
            queryset = queryset.filter(deleted=False)
        return get_actions(queryset, self.request.GET.dict(), self.team_id)
    def list(self, request: request.Request, *args: Any, **kwargs: Any) -> Response:
        actions = self.get_queryset()
        actions_list: List[Dict[Any, Any]] = self.serializer_class(actions, many=True, context={"request": request}).data  
        if request.GET.get("include_count", False):
            actions_list.sort(key=lambda action: action.get("count", action["id"]), reverse=True)
        return Response({"results": actions_list})
    @action(methods=["GET"], detail=False)
    def trends(self, request: request.Request, *args: Any, **kwargs: Any) -> Response: