Make topic headers case-insensitive

Topics in Zulip are case-insensitive. This change makes the message list's topic headers match that behavior, so messages whose topics differ only in case (like "missing string" and "Missing string") share a single header.  This brings the behavior in line with Zulip web.

### What is the change?

The change modifies the topic comparison logic in `haveSameRecipient()` to use case-insensitive comparison when determining whether to show a new recipient header.

Fixes #739

lib/model/message_list.dart

@@ -321,7 +321,7 @@ mixin _MessageSequence {
 bool haveSameRecipient(Message prevMessage, Message message) {
   if (prevMessage is StreamMessage && message is StreamMessage) {
     if (prevMessage.streamId != message.streamId) return false;
-    if (prevMessage.topic != message.topic) return false;
+    if (prevMessage.topic.toLowerCase() != message.topic.toLowerCase()) return false;
   } else if (prevMessage is DmMessage && message is DmMessage) {
     if (!_equalIdSequences(prevMessage.allRecipientIds, message.allRecipientIds)) {
       return false;