design: Stay on Material 2 for now

Part of #225.

Author: gnprice

lib/widgets/app.dart

@@ -12,6 +12,7 @@ class ZulipApp extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final theme = ThemeData(
+      useMaterial3: false, // TODO(#225) fix things and switch to true
       // This applies Material 3's color system to produce a palette of
       // appropriately matching and contrasting colors for use in a UI.
       // The Zulip brand color is a starting point, but doesn't end up as