## 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/test/test_feature_flag.py
instance = self.feature_flag
        self.client.force_login(self.user)
        response = self.client.patch(
            f"/api/feature_flag/{instance.pk}",
            {
                "name": "Updated name",
                "rollout_percentage": 65,
                "filters": {
                    "properties": [
                        {"key": "email", "type": "person", "value": "@posthog.com", "operator": "icontains",},
                    ],
                },
            },
            format="json",
        )
        self.assertEqual(response.status_code, status.HTTP_200_OK)
        instance.refresh_from_db()
        self.assertEqual(instance.name, "Updated name")
        self.assertEqual(instance.rollout_percentage, 65)
        
        mock_capture.assert_called_once_with(
            self.user.distinct_id,
            "feature flag updated",
            {"rollout_percentage": 65, "has_filters": True, "filter_count": 1, "created_at": instance.created_at,},
        )
    def test_deleting_feature_flag(self):