Put group calling behind a remote feature flag

ts/util/isGroupCallingEnabled.ts

@@ -0,0 +1,13 @@
+// Copyright 2020 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { isBeta } from './version';
+import * as RemoteConfig from '../RemoteConfig';
+
+// We can remove this function once group calling has been turned on for everyone.
+export function isGroupCallingEnabled(): boolean {
+  return (
+    RemoteConfig.isEnabled('desktop.groupCalling') ||
+    isBeta(window.getVersion())
+  );
+}