Private Email Service Support (#1028)

* Added ServerSettingKey's for SMTP and moved email service code to Kavita. Nothing integrated in the UI yet.

* Undo all the custom SMTP stuff and prepare for custom email service url.

* Foundation for email service to use a custom url is setup.

* Implemented the ability to hook up custom email url

API/Entities/Enums/ServerSettingKey.cs

@@ -71,6 +71,10 @@ namespace API.Entities.Enums
         /// </summary>
         [Description("BookmarkDirectory")]
         BookmarkDirectory = 12,
-
+        /// <summary>
+        /// If SMTP is enabled on the server
+        /// </summary>
+        [Description("CustomEmailService")]
+        EmailServiceUrl = 13,
     }
 }