msglist: Fix mark-as-read button icon color in dark mode

After sweeping through the message-list screen to implement dark
theme (zulip#843), in late July, the Flutter framework merged
flutter/flutter#143501, in early August. This made it so our
`foregroundColor` no longer controlled the icon color, as promised
in the dartdocs of `foregroundColor` and `iconColor`. I commented
about this on the PR:
  flutter/flutter#143501 (comment)

Fixes: zulip#926

lib/widgets/message_list.dart

@@ -758,6 +758,7 @@ class _MarkAsReadWidgetState extends State<MarkAsReadWidget> {
                 // state is disabled, pressed, etc.  We handle those states
                 // separately, via MarkAsReadAnimation.
                 foregroundColor: const WidgetStatePropertyAll(Colors.white),
+                iconColor: const WidgetStatePropertyAll(Colors.white),
                 backgroundColor: WidgetStatePropertyAll(messageListTheme.unreadMarker),
               ),
               onPressed: _loading ? null : () => _handlePress(context),