home: Add switch account icon in main menu

Fixes #1178.

Author: rishichirchi

lib/widgets/home.dart

@@ -554,11 +554,7 @@ class _SwitchAccountButton extends _MenuButton {
   const _SwitchAccountButton();
 
   @override
-  // TODO(design): choose an icon
-  IconData? get icon => null;
-
-  @override
-  Widget buildLeading(BuildContext context) => const SizedBox.shrink();
+  IconData? get icon => ZulipIcons.arrow_left_right;
 
   @override
   String label(ZulipLocalizations zulipLocalizations) {