store [nfc]: Move zulipFeatureLevel to PerAccountStoreBase

Author: PIG208

lib/model/store.dart

@@ -372,6 +372,10 @@ abstract class PerAccountStoreBase {
 
   ApiConnection get connection => _core.connection;
 
+  /// Always equal to `connection.zulipFeatureLevel`
+  /// and `account.zulipFeatureLevel`.
+  int get zulipFeatureLevel => connection.zulipFeatureLevel!;
+
   String get queueId => _core.queueId;
 
   ////////////////////////////////
@@ -554,10 +558,6 @@ class PerAccountStore extends PerAccountStoreBase with ChangeNotifier, EmojiStor
   ////////////////////////////////
   // Data attached to the realm or the server.
 
-  /// Always equal to `connection.zulipFeatureLevel`
-  /// and `account.zulipFeatureLevel`.
-  int get zulipFeatureLevel => connection.zulipFeatureLevel!;
-
   String get zulipVersion => account.zulipVersion;
   final RealmWildcardMentionPolicy realmWildcardMentionPolicy; // TODO(#668): update this realm setting
   final bool realmMandatoryTopics;  // TODO(#668): update this realm setting