compose: Use maxTopicLength instead of the hardcoded value

Fixes zulip#307

Author: AhmedTareek

lib/api/route/messages.dart

@@ -155,8 +155,6 @@ class GetMessagesResult {
   Map<String, dynamic> toJson() => _$GetMessagesResultToJson(this);
 }
 
-// https://zulip.com/api/send-message#parameter-topic
-const int kMaxTopicLengthCodePoints = 60;
 
 // https://zulip.com/api/send-message#parameter-content
 const int kMaxMessageLengthCodePoints = 10000;

lib/widgets/compose_box.dart

@@ -143,6 +143,7 @@ enum TopicValidationError {
 
 class ComposeTopicController extends ComposeController<TopicValidationError> {
   ComposeTopicController({required this.store}) {
+    maxLengthUnicodeCodePoints = store.maxTopicLength;
     _update();
   }
 
@@ -151,8 +152,7 @@ class ComposeTopicController extends ComposeController<TopicValidationError> {
   // TODO(#668): listen to [PerAccountStore] once we subscribe to this value
   bool get mandatory => store.realmMandatoryTopics;
 
-  // TODO(#307) use `max_topic_length` instead of hardcoded limit
-  @override final maxLengthUnicodeCodePoints = kMaxTopicLengthCodePoints;
+  @override late int maxLengthUnicodeCodePoints;
 
   @override
   String _computeTextNormalized() {