## 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
response = self.client.patch(
            "/api/feature_flag/%s/" % another_feature_flag.pk,
            data={"name": "Beta feature", "key": "beta-feature"},
            content_type="application/json",
        ).json()
        self.assertEqual(
            response,
            {"type": "validation_error", "code": "key-exists", "detail": "This key already exists.", "attr": None},
        )
        
        response = self.client.patch(
            "/api/feature_flag/%s/" % feature_flag["id"],
            data={"name": "Beta feature 3", "key": "beta-feature"},
            content_type="application/json",
        )
        self.assertEqual(response.status_code, 200)
        self.assertEqual(FeatureFlag.objects.get(pk=feature_flag["id"]).name, "Beta feature 3")
    def test_is_simple_flag(self):
        feature_flag = self.client.post(
            "/api/feature_flag/",
            data={
                "name": "Beta feature",
                "key": "beta-feature",
                "rollout_percentage": 50,
                "filters": {"properties": [{"key": "email", "value": "tim@posthog.com"}]},
            },
            content_type="application/json",
        ).json()
        self.assertFalse(feature_flag["is_simple_flag"])
class TestAPIFeatureFlag(APIBaseTest):