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

Kavita.Common/Helpers/UntrustedCertClientFactory.cs

@@ -0,0 +1,13 @@
+using System.Net.Http;
+using Flurl.Http.Configuration;
+
+namespace Kavita.Common.Helpers;
+
+public class UntrustedCertClientFactory : DefaultHttpClientFactory
+{
+    public override HttpMessageHandler CreateMessageHandler() {
+        return new HttpClientHandler {
+            ServerCertificateCustomValidationCallback = (_, _, _, _) => true
+        };
+    }
+}