## 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 



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posthog/api/feature_flag.py
from typing import Any, Dict
from django.db import IntegrityError
from django.db.models import QuerySet
from rest_framework import serializers, viewsets
from rest_framework.permissions import IsAuthenticated
from posthog.api.routing import StructuredViewSetMixin
from posthog.api.user import UserSerializer
from posthog.mixins import AnalyticsDestroyModelMixin
from posthog.models import FeatureFlag
from posthog.permissions import ProjectMembershipNecessaryPermissions
class FeatureFlagSerializer(serializers.HyperlinkedModelSerializer):