Replace old markdown and remove redundant events (#389)

* Remove redundant messages

* Replaced old markdown with preview.
Added nuget config to add preview feed

* Fix old extensions usage

* Scrollbar margin for the markdown.

---------

Co-authored-by: Burak Kaan Köse <bkaankose@outlook.com>

Wino.Mail.ViewModels/Messages/ResetSingleMailItemSelectionEvent.cs

@@ -1,19 +0,0 @@
-using Wino.Mail.ViewModels.Data;
-
-namespace Wino.Mail.ViewModels.Messages
-{
-
-    /// <summary>
-    /// When a thread conversation listview has single selection, all other listviews
-    /// must unselect all their items.
-    /// </summary>
-    public class ResetSingleMailItemSelectionEvent
-    {
-        public ResetSingleMailItemSelectionEvent(MailItemViewModel selectedViewModel)
-        {
-            SelectedViewModel = selectedViewModel;
-        }
-
-        public MailItemViewModel SelectedViewModel { get; set; }
-    }
-}