## 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
def setUp(self):
        super().setUp()
        self.organization, self.team, self.user = User.objects.bootstrap("Feature Flags", "ff@posthog.com", None)
        self.feature_flag = FeatureFlag.objects.create(team=self.team, created_by=self.user, key="red_button",)
    @patch("posthoganalytics.capture")
    def test_creating_feature_flag(self, mock_capture):
        self.client.force_login(self.user)
        response = self.client.post(
            "/api/feature_flag/", {"name": "Alpha feature", "key": "alpha-feature", "rollout_percentage": 50,},
        )
        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
        instance = FeatureFlag.objects.get(id=response.data["id"])  
        self.assertEqual(instance.key, "alpha-feature")
        
        mock_capture.assert_called_once_with(
            self.user.distinct_id,
            "feature flag created",
            {"rollout_percentage": 50, "has_filters": False, "filter_count": 0, "created_at": instance.created_at,},
        )
    @patch("posthoganalytics.capture")
    def test_updating_feature_flag(self, mock_capture):