## Bug description
Feature Flag cannot recreate with the same key after deleting 

Steps to reproduce: 

1. create feature flag with key `bla`
2. delete the feature flag
3. try to create again feature flag with `bla` key
4. alert will popup with This key already exists
![image](https://user-images.githubusercontent.com/24251369/99444738-4e5a4680-2925-11eb-822a-5a9397fba965.png)


## Expected behavior
New feature flag will be added

## Environment

self-hosting 



#### *Thank you* for your bug report – we love squashing them!

posthog/api/feature_flag.py
request = self.context["request"]
        validated_data["created_by"] = request.user
        validated_data["team_id"] = self.context["team_id"]
        try:
            feature_flag = super().create(validated_data)
        except IntegrityError:
            raise serializers.ValidationError("This key already exists.", code="key-exists")
        return feature_flag
    def update(self, instance: FeatureFlag, validated_data: Dict, *args: Any, **kwargs: Any) -> FeatureFlag:  # 
        try:
            return super().update(instance, validated_data)
        except IntegrityError:
            raise serializers.ValidationError("This key already exists.", code="key-exists")
class FeatureFlagViewSet(StructuredViewSetMixin, AnalyticsDestroyModelMixin, viewsets.ModelViewSet):
    legacy_team_compatibility = True  # 
    queryset = FeatureFlag.objects.all()
    serializer_class = FeatureFlagSerializer
    permission_classes = [IsAuthenticated, ProjectMembershipNecessaryPermissions]
    def get_queryset(self) -> QuerySet:
        queryset = super().get_queryset()
        if self.action == "list":
            queryset = queryset.filter(deleted=False)
        return queryset.order_by("-created_at")