Embedded images replaced with cid linked resources. (#313)

* Added logic to replace embedded images with linked resources

* Added alt text for images and replaced NewtonSoft with Text.Json

* Fix draft mime preparation

* Fix crashes for signatures without images.

---------

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

Wino.Core/Services/MailService.cs

@@ -636,10 +636,7 @@ namespace Wino.Core.Services
                 _ => CreateReferencedDraft(builder, message, draftCreationOptions, account, signature),
             };
 
-            if (!string.IsNullOrEmpty(builder.HtmlBody))
-            {
-                builder.TextBody = HtmlAgilityPackExtensions.GetPreviewText(builder.HtmlBody);
-            }
+            builder.SetHtmlBody(builder.HtmlBody);
 
             message.Body = builder.ToMessageBody();