msglist [nfc]: Pull out a _showAsMuted method

This condition is a bit complex already, and will become more so
soon.  So making it a method instead of an expression gives its logic
a bit more room to breathe and spread out.

Author: gnprice

lib/widgets/message_list.dart

@@ -1894,6 +1894,14 @@ class _SenderRow extends StatelessWidget {
   final MessageBase message;
   final bool showTimestamp;
 
+  bool _showAsMuted(BuildContext context, PerAccountStore store) {
+    final message = this.message;
+    if (!store.isUserMuted(message.senderId)) return false;
+    if (message is! Message) return false; // i.e., if an outbox message
+    return !MessageListPage.revealedMutedMessagesOf(context)
+      .isMutedMessageRevealed(message.id);
+  }
+
   @override
   Widget build(BuildContext context) {
     final store = PerAccountStoreWidget.of(context);
@@ -1904,10 +1912,7 @@ class _SenderRow extends StatelessWidget {
     final time = _kMessageTimestampFormat
       .format(DateTime.fromMillisecondsSinceEpoch(1000 * message.timestamp));
 
-    final showAsMuted = store.isUserMuted(message.senderId)
-      && message is Message // i.e., not an outbox message
-      && !MessageListPage.revealedMutedMessagesOf(context)
-                          .isMutedMessageRevealed((message as Message).id);
+    final showAsMuted = _showAsMuted(context, store);
 
     return Padding(
       padding: const EdgeInsets.fromLTRB(16, 2, 16, 0),