webhooks/github: Ignore repository_vulnerability_alert event.

This event isn't incredibly common/useful and errors for this
event were cluttering up our webhook logs.

zerver/webhooks/github/fixtures/repository_vulnerability_alert.json

@@ -0,0 +1,33 @@
+{
+  "action": "dismiss",
+  "alert": {
+    "id": 7649605,
+    "affected_range": "0.2.0",
+    "affected_package_name": "many_versioned_gem",
+    "external_reference": "https://nvd.nist.gov/vuln/detail/CVE-2018-3728",
+    "external_identifier": "CVE-2018-3728",
+    "fixed_in": "0.2.5",
+    "dismisser": {
+      "login":"octocat",
+      "id":1,
+      "node_id": "MDQ6VXNlcjIxMDMxMDY3",
+      "avatar_url":"https://github.com/images/error/octocat_happy.gif",
+      "gravatar_id":"",
+      "url":"https://api.github.com/users/octocat",
+      "html_url":"https://github.com/octocat",
+      "followers_url":"https://api.github.com/users/octocat/followers",
+      "following_url":"https://api.github.com/users/octocat/following{/other_user}",
+      "gists_url":"https://api.github.com/users/octocat/gists{/gist_id}",
+      "starred_url":"https://api.github.com/users/octocat/starred{/owner}{/repo}",
+      "subscriptions_url":"https://api.github.com/users/octocat/subscriptions",
+      "organizations_url":"https://api.github.com/users/octocat/orgs",
+      "repos_url":"https://api.github.com/users/octocat/repos",
+      "events_url":"https://api.github.com/users/octocat/events{/privacy}",
+      "received_events_url":"https://api.github.com/users/octocat/received_events",
+      "type":"User",
+      "site_admin":true
+    },
+    "dismiss_reason": "No bandwidth to fix this",
+    "dismissed_at": "2017-10-25T00:00:00+00:00"
+  }
+}

zerver/webhooks/github/tests.py

@@ -362,3 +362,14 @@ class GithubWebhookTest(WebhookTestCase):
         result = self.client_post(self.url, payload, HTTP_X_GITHUB_EVENT='push', content_type="application/json")
         self.assertFalse(check_send_webhook_message_mock.called)
         self.assert_json_success(result)
+
+    @patch('zerver.webhooks.github.view.check_send_webhook_message')
+    def test_repository_vulnerability_alert_ignore(
+            self, check_send_webhook_message_mock: MagicMock) -> None:
+        self.url = self.build_webhook_url()
+        payload = self.get_body('repository_vulnerability_alert')
+        result = self.client_post(self.url, payload,
+                                  HTTP_X_GITHUB_EVENT='repository_vulnerability_alert',
+                                  content_type="application/json")
+        self.assertFalse(check_send_webhook_message_mock.called)
+        self.assert_json_success(result)

zerver/webhooks/github/view.py

@@ -423,6 +423,10 @@ EVENT_FUNCTION_MAPPER = {
     'watch': get_watch_body,
 }
 
+IGNORED_EVENTS = [
+    'repository_vulnerability_alert'
+]
+
 @api_key_only_webhook_view('GitHub', notify_bot_owner_on_invalid_json=True)
 @has_request_variables
 def api_github_webhook(
@@ -470,6 +474,8 @@ def get_event(request: HttpRequest, payload: Dict[str, Any], branches: str) -> O
             return "push_tags"
     elif event in list(EVENT_FUNCTION_MAPPER.keys()) or event == 'ping':
         return event
+    elif event in IGNORED_EVENTS:
+        return None
 
     raise UnexpectedWebhookEventType('GitHub', event)