## 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
new_user = User.objects.create_and_join(self.organization, "new_annotations@posthog.com", None)
        instance = FeatureFlag.objects.create(team=self.team, created_by=self.user)
        self.client.force_login(new_user)
        with patch("posthoganalytics.capture") as mock_capture:
            response = self.client.delete(f"/api/feature_flag/{instance.pk}/")
        self.assertEqual(response.status_code, status.HTTP_204_NO_CONTENT)
        self.assertFalse(FeatureFlag.objects.filter(pk=instance.pk).exists())
        
        mock_capture.assert_called_once_with(
            new_user.distinct_id,
            "feature flag deleted",
            {"rollout_percentage": None, "has_filters": False, "filter_count": 0, "created_at": instance.created_at,},
        )
    @patch("posthoganalytics.capture")
    def test_cannot_delete_feature_flag_on_another_team(self, mock_capture):
        organization, team, user = User.objects.bootstrap("Test", "team2@posthog.com", None)
        self.client.force_login(user)
        response = self.client.delete(f"/api/feature_flag/{self.feature_flag.pk}/")
        self.assertEqual(response.status_code, status.HTTP_404_NOT_FOUND)
        self.assertTrue(FeatureFlag.objects.filter(pk=self.feature_flag.pk).exists())
        mock_capture.assert_not_called()