Fix render loop in <Modal>, clean up ref merger code

ts/hooks/useRefMerger.ts

@@ -0,0 +1,8 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { useMemo } from 'react';
+import { createRefMerger } from '../util/refMerger';
+
+export const useRefMerger = (): ReturnType<typeof createRefMerger> =>
+  useMemo(createRefMerger, []);