Allow to create new choices on survey on its creation

lib/models/new_post.dart

@@ -12,12 +12,15 @@ import 'package:meta/meta.dart';
 class NewSurvey {
   final String question;
   final Set<String> answers;
+  final bool allowNewChoicesCreation;
 
   const NewSurvey({
     @required this.question,
     @required this.answers,
+    @required this.allowNewChoicesCreation,
   })  : assert(question != null),
-        assert(answers.length > 1);
+        assert(answers.length > 1),
+        assert(allowNewChoicesCreation != null);
 }
 
 class NewPost {