In <ContactPills>, use new "scrollToBottom" helper

ts/util/scrollToBottom.ts

@@ -0,0 +1,8 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+export function scrollToBottom(el: HTMLElement): void {
+  // We want to mutate the parameter here.
+  // eslint-disable-next-line no-param-reassign
+  el.scrollTop = el.scrollHeight;
+}