msglist [nfc]: Have MessageWithSender take the whole message-item object

This will make things more convenient when we add another field
on these which MessageWithSender will want to consult.

Author: gnprice

lib/widgets/message_list.dart

@@ -5,7 +5,6 @@ import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
 
 import '../api/model/model.dart';
-import '../model/content.dart';
 import '../model/message_list.dart';
 import '../model/narrow.dart';
 import '../model/store.dart';
@@ -391,7 +390,7 @@ class MessageItem extends StatelessWidget {
       child: Column(children: [
         DecoratedBox(
           decoration: borderDecoration,
-          child: MessageWithSender(message: message, content: item.content)),
+          child: MessageWithSender(item: item)),
         if (trailing != null && item.isLastInBlock) trailing!,
       ]));
 
@@ -564,14 +563,13 @@ class RecipientHeaderChevronContainer extends StatelessWidget {
 
 /// A Zulip message, showing the sender's name and avatar.
 class MessageWithSender extends StatelessWidget {
-  const MessageWithSender(
-    {super.key, required this.message, required this.content});
+  const MessageWithSender({super.key, required this.item});
 
-  final Message message;
-  final ZulipContent content;
+  final MessageListMessageItem item;
 
   @override
   Widget build(BuildContext context) {
+    final message = item.message;
     final time = _kMessageTimestampFormat
       .format(DateTime.fromMillisecondsSinceEpoch(1000 * message.timestamp));
 
@@ -601,7 +599,7 @@ class MessageWithSender extends StatelessWidget {
                   child: Text(message.senderFullName, // TODO get from user data
                     style: const TextStyle(fontWeight: FontWeight.bold))),
                 const SizedBox(height: 4),
-                MessageContent(message: message, content: content),
+                MessageContent(message: message, content: item.content),
               ])),
           Container(
             width: 80,