Fix build by adding IsReadOnly implementation

Author: jcansdale

src/GitHub.InlineReviews/ViewModels/TooltipCommentViewModel.cs

@@ -28,6 +28,8 @@ internal TooltipCommentViewModel(IAccount user, string body, DateTimeOffset upda
 
         public int Id { get; }
 
+        public bool IsReadOnly { get; set; }
+
         public DateTimeOffset UpdatedAt { get; }
 
         public IAccount User { get; }