Different message for admin deletion, check not to delete last admin

Ref: #549, #550

BTCPayServer/Models/ConfirmModel.cs

@@ -7,6 +7,15 @@ namespace BTCPayServer.Models
 {
     public class ConfirmModel
     {
+        public ConfirmModel() { }
+
+        public ConfirmModel(string title, string desc, string action = null)
+        {
+            Title = title;
+            Description = desc;
+            Action = action;
+        }
+
         public string Title
         {
             get; set;