intercom: Return success on HEAD requests.

Intercom sends a HEAD request to validate the webhook URL on their side,
which was not expected in the previous implementation.
This fixes the problem that we send out a confusing error message for it.

Fixes #23912.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

zerver/webhooks/intercom/tests.py

@@ -265,3 +265,7 @@ New user created:
             "Contact: Eeshan Garg",
             "User unsubscribed from emails.",
         )
+
+    def test_success_on_http_head(self) -> None:
+        result = self.client_head(self.url)
+        self.assertEqual(result.status_code, 200)