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/Data/Seed.cs

@@ -60,6 +60,7 @@ namespace API.Data
                 new () {Key = ServerSettingKey.InstallId, Value = HashUtil.AnonymousToken()},
                 new () {Key = ServerSettingKey.InstallVersion, Value = BuildInfo.Version.ToString()},
                 new () {Key = ServerSettingKey.BookmarkDirectory, Value = directoryService.BookmarkDirectory},
+                new () {Key = ServerSettingKey.EmailServiceUrl, Value = EmailService.DefaultApiUrl},
             };
 
             foreach (var defaultSetting in DefaultSettings)