[BUG] Disable CODEOWNERS for forked repositories

- Disable the CODEOWNERS feature for forked repositories, as it would
otherwise inadvertently request reviewers when for example a pull
request is opened against a forked repository to propose changes to an
existant pull request in the original repository.
- Adds integration test.
- Resolves #2525

models/issues/pull.go

@@ -891,6 +891,14 @@ func PullRequestCodeOwnersReview(ctx context.Context, pull *Issue, pr *PullReque
 		return nil
 	}
 
+	if err := pull.LoadRepo(ctx); err != nil {
+		return err
+	}
+
+	if pull.Repo.IsFork {
+		return nil
+	}
+
 	if err := pr.LoadBaseRepo(ctx); err != nil {
 		return err
 	}