added guardian letters with chat and multiple answer functionalities

lib/storage/notification_settings.dart

@@ -16,9 +16,20 @@ class NotificationSettings {
   @JsonKey(defaultValue: false)
   bool talkPermissionPromptShown;
 
+  /// Same one-shot guards for the two occasions on which accounts with
+  /// parent letters are asked: right after signing in and, if the permission
+  /// is still missing, on the first visit of the parent letters.
+  @JsonKey(defaultValue: false)
+  bool guardianLoginPromptShown;
+
+  @JsonKey(defaultValue: false)
+  bool parentLettersPromptShown;
+
   NotificationSettings({
     this.enabled = true,
     this.talkPermissionPromptShown = false,
+    this.guardianLoginPromptShown = false,
+    this.parentLettersPromptShown = false,
   });
 
   factory NotificationSettings.fromJson(Map<String, dynamic> json) =>