msglist: Pass last message id to topic action sheet instead of first

In the next commit(s), when creating topic permalinks, we try to use
max message id of the topic, if possible. If we cannot find the max
message id, then I think better to use a message id near to max message id.

Related CZO discussions:
  https://chat.zulip.org/#narrow/channel/101-design/topic/redirects.20from.20near.20links/near/1460694
  https://chat.zulip.org/#narrow/channel/101-design/topic/redirects.20from.20near.20links/near/1774403

Author: sm-sayedi

lib/widgets/message_list.dart

@@ -584,7 +584,7 @@ class MessageListAppBarTitle extends StatelessWidget {
               behavior: HitTestBehavior.translucent,
               onLongPress: () {
                 final someMessage = MessageListPage.ancestorOf(context)
-                  .model?.messages.firstOrNull;
+                  .model?.messages.lastOrNull;
                 // If someMessage is null, the topic action sheet won't have a
                 // resolve/unresolve button. That seems OK; in that case we're
                 // either still fetching messages (and the user can reopen the