store [nfc]: Move store.sendMessage up near other MessageStore proxy impls

Thanks Greg for noticing this:
  zulip#1484 (comment)

Author: chrisbobbe

lib/model/store.dart

@@ -737,6 +737,11 @@ class PerAccountStore extends PerAccountStoreBase with ChangeNotifier, EmojiStor
   void unregisterMessageList(MessageListView view) =>
     _messages.unregisterMessageList(view);
   @override
+  Future<void> sendMessage({required MessageDestination destination, required String content}) {
+    assert(!_disposed);
+    return _messages.sendMessage(destination: destination, content: content);
+  }
+  @override
   void reconcileMessages(List<Message> messages) {
     _messages.reconcileMessages(messages);
     // TODO(#649) notify [unreads] of the just-fetched messages
@@ -904,12 +909,6 @@ class PerAccountStore extends PerAccountStoreBase with ChangeNotifier, EmojiStor
     }
   }
 
-  @override
-  Future<void> sendMessage({required MessageDestination destination, required String content}) {
-    assert(!_disposed);
-    return _messages.sendMessage(destination: destination, content: content);
-  }
-
   static List<CustomProfileField> _sortCustomProfileFields(List<CustomProfileField> initialCustomProfileFields) {
     // TODO(server): The realm-wide field objects have an `order` property,
     //   but the actual API appears to be that the fields should be shown in