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/Helpers/Converters/ServerSettingConverter.cs

@@ -42,6 +42,9 @@ namespace API.Helpers.Converters
                     case ServerSettingKey.BookmarkDirectory:
                         destination.BookmarksDirectory = row.Value;
                         break;
+                    case ServerSettingKey.EmailServiceUrl:
+                        destination.EmailServiceUrl = row.Value;
+                        break;
                 }
             }