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/AutoDiscoveryService.cs

@@ -1,7 +1,7 @@
 using System;
 using System.Net.Http;
+using System.Text.Json;
 using System.Threading.Tasks;
-using Newtonsoft.Json;
 using Serilog;
 using Wino.Core.Domain.Interfaces;
 using Wino.Core.Domain.Models.AutoDiscovery;
@@ -43,7 +43,7 @@ namespace Wino.Core.Services
             {
                 var content = await response.Content.ReadAsStringAsync();
 
-                return JsonConvert.DeserializeObject<AutoDiscoverySettings>(content);
+                return JsonSerializer.Deserialize<AutoDiscoverySettings>(content);
             }
             catch (Exception ex)
             {